In a previous question, I asked why a taboo would develop around a food that grows well in a civilization’s climate and has no harmful effects. However, I will now ask the opposite question: why would a civilization embrace a food that does not grow well in its environment when other foods that are equally tasty and have roughly equivalent nutritional properties grow well in its climate?
Your question isn’t a hypothetical, it’s a serious real world issue.
Beef production in the western world has extremely negative impacts on our environment due to massive methane emissions, deforestation and disproportionate water use. (https://www.nationalgeographic.com/distribution/public/amp/environment/2019/01/commission-report-great-food-transformation-plant-diet-climate-change)
So why do people continue to eat a very environmentally destructive product by the billions of tons? Simple, it’s tasty and it was sustainable back when cows were being moved over range rather than being force fed corn and soy that people could eat instead.
So perhaps you have a crop that is extremely water intensive, and this was fine before the climate started to change, but now semi annual droughts are preventing the aquifers from being refilled, so the crop’s water consumption is now unsustainable and is drastically lowering the available water. But people refuse to adapt because it’s something they make money on and they love the taste.
TL;DR: Just take a look at climate change and food, especially in California
There are several reasons why one food would be preferred over equally nutritious alternatives. They might apply singularly or in groups.
Historical Preference: The food could hold a strong place in the cultural identity of members of the society. The Coca Cola of my fore-fathers as it were or All True Hurkeshmen eat Blarn!
Emblem of Wealth and Status: Consumption sends a social cue that that you are rich and powerful. Robin Leech eating caviar on Lifestyles of the Rich and Famous sort of thing
Signifies Sanctity or Religious Purity or Sexual Purity: The holier than thou eat the stuff to show their ways are the old ways.
Fashionable: Eating it shows just how down with your bad self you really are. It demonstrates that you are one fruggy drude who knows where her towel is.
The crop is the only thing the people there can grow
As other people have pointed out this is a very real problem with large ungulate farming in many places across the world. What they haven’t pointed out are some of the other reasons why it keeps happening despite being environmentally detrimental. In some cases despite being negative on a global level the crops or livestock in question are the only thing that can reliably grow there.
Case in point in the U.S. there is a region in North America west of the 100th meridian west where there is very little water. Crops grow very poorly here, and are either dependent on widescale irrigation or are restricted to the few rivers and streams around there. Trying to farm there has actually screwed the environment up worse than cattle ranching, increasing desertification (the line previously at the 100th meridian is moving east and the Ogallala aquifer is messed up) and is screwing up the Colorado River. About the only thing you can reasonably do with the land is raise sheep and cattle, otherwise it’s useless for settlement. Big ungulates basically fill the niche that bison used to, though it does nothing about the methane emission problems.
Australia is in a similar situation. Australia historically had a huge sheep industry, but sheep are problematic because they are descended from mountain ungulates and so tend to strip land bare in search of food. This is why sheep farmers and other livestock farmers often do not get along, to the point of armed conflict and murder between the two. The fact that sheep get the maximum use out of plant matter makes them good at surviving in the Outback, but it also means they tend to destroy the environment. This is really bad for Australia, whose ecosystem relies on native vegetation keeping water locked up on land. Australia’s ecosystem is screwed up due to sheep farming and the desertification it causes, and a lot of the problems blamed on foxes, cats, and rabbits are at least partly due to livestock raising (e.g., providing water and food for invasive pests, accidentally making the ecosystem more favorable for them, clearing out brush the native animals need to hide in, people killing native predators for fear that they eat sheep, sheep stripping the ground bare leaving the native animals little to eat and increasingly fewer places to go). However at this point outright banning sheep farming in Australia would probably result in the country starving (either directly or due to the economy crashing).
It's a cash crop
This is a big problem in California. The state has a semi-arid Mediterranean climate which makes it good for year-round farming but comes with a drawback of a very limited water supply. Despite the droughts, a few years ago (unsure if it is still the case) almonds started getting popular due to the high demand for them, and a lot of farmers started switching from less water-intensive crops to almonds. This, unsurprisingly, made the water issues worse.
They claim it as part of their cultural heritage
I.e., whaling and Japan and Norway. Whaling is definitely detrimental to the environment, a lot of the species hunted are endangered, it's definitely not sustainable in the case of species like right whales, and the average person in these countries doesn't even seem to like eating whale meat, the demand is so low the Japanese government has started selling whale meat in school cafeterias (and when that happens you know demand is low).
Mostly the reason these countries do it is they claim it is part of their cultural heritage, and there is a lot of pressure from the governments to keep it up. One of the few things that seems to get the average person in these countries to support whaling is ironically pressure from other countries, because it makes them feel like outsiders are trying to tell them what to do and erase their culture.
Other examples of unsustainable agriculture or hunting include farming salmon in Chile, and a lot of aquaculture in general. Bluefin tuna aquaculture actually makes their situation worse because they remove juveniles from the environment instead of actually raising them in captivity, but demand is so high it's unlikely to stop.
This isn’t just a modern civilization thing, the Mayans built their civilization on the very limited water supply of the cenotes surrounding the Chicxulub crater, the Garamantians relied on “fossil water” from non-refilling springs in north Africa and when the springs dried up they disappeared, there are thoughts that some of the Mesopotamian civilization crashed when they farmed the area around the Tigris and Euphrates so hard it built up salt and farming efficiency ceased. Humans have a habit of trying to grow food in risky places and subsequently getting bitten by the consequences.
The rarity factor alone is enough.
Beef doesn't necessarily taste any better than pork, but if you eat pork all the time, then beef becomes a rare and sought-after delicacy. Socially, those able to afford it will serve expensive beef at their feasts to signify their superior status, and look down on pork-eaters, regardless of which is better or more nutritious.
Novelty in diet is extremely important, as any restaurateur will tell, you -- even McD's and KFS keep shifting their menu around.
In a far more appalling real-life example, many people in West Africa drink expensive imported colas with negligible nutritional value rather than the cheap, vitamin-packed fruit juice from fruits that grow well locally. The multinationals profit, the local fruit growers struggle, and it doesn't do any good for children's health. It's not rational, it's real life.
I'm going to drop this here first: https://www.mcdonalds.com/
Yes that wasn't your question, but I thought it was humorous - imagine if your world had burger trees that were rare and would definitely make you fat, but still became a delicacy.
Anyhow, there's a few reasons for it:
- It's rare: That's it. That's the selling point. If it's rare, only the rich can have it.
- Symbolic: Maybe this plant is rare, but at the same time it is not only rare in your region, but in the world. In fact, this mystery plant only exists in your nation (for whatever reason). You don't have bounties of this plant, but its rarity is interpreted as significant by the people, and are reserved only for the holiest/elite people in the nation.
- Difficult terrain/difficult to grow: Maybe this plant only grows when it rains over an active volcano as lightning strikes exactly once a foot away from it. Is this oddly specific? Yes, and for no reason. This plant is ridiculously hard to find and is rumored to have special properties. Your sorcerers and scientists argue that this plant is a freak of nature but has nothing special about it, but all three of those things combined is really cool so hey why not - people are into it now.
- I owe my nation to this plant: Athena's gift to the Athenians was the olive tree. Poseidon's was a salty spring. One of those was useful, and gained the favor of the city-state. Maybe up the stakes a little bit - this plant is something all of your nations are afraid of (maybe they're allergic or something). Even if this plant is completely insignificant/inconsequential for your people, if it hurts your enemies it is definitely important.
- Basic tourism things: You have to try this dish/see this plant - you can only find it here! This plant is a tourist trap. Maybe your locals don't care about it but hey it lets you sell over-expensive hiking trips to see 3 of these rare and mostly useless plants. Pineapples, for example, are not found in Europe, and are not needed for a person to survive. Didn't stop it from becoming a souvenir people kept trying to bring back from the colonies. Was also a symbol of wealth that way.
Throw some magic in these explanations and you'll have even better answers. Hope this helps!