Colonization is expensive
It takes a lot of people and resources to get a colony going. You'll have to provide the people with everything they need to survive while they are getting started/settled. Prepared food, livestock, seeds, tools, pre-fab shelter (tents), weaponry, transportation, and the list goes on.
Then, if there are any monsters, predator animals, insects, etc. (as suggested in other answers), you'll be defending the colonists against all that at the same time as trying to set up livestock pens, more permanent housing, farms, a fortified structure, and all the other things even a basic village needs. This means more weapons, replacement warriors, more transportation (boats), more food, etc. (again).
You'll also likely have to defend against raids of other countries that believe that if they can't afford to setup a colony, neither should you. Besides, it's a soft spot in your defenses while it's still getting set up, draining your resources and distracting you from a possible attack against somewhere else on your homeland.
Even after the colony is somewhat setup, it'll still likely be a soft spot, since
it's harder to get your armies there, due to needing boats (again), than it is to simply march them somewhere on your main island.
It could take a decade (or more) to get a real handhold on this one tiny spot on the "new" island, and there may be people who eventually give up and move back, so you'll need to provide transport back, while also transporting more people as replacements.
This is a Lot of work, time, expense, and that's even if you sent out scouts ahead of time to find a good spot that's easily defensible, has good land for crops and grazing, a good source of clean water, and isn't a gathering spot for the Nasties mentioned earlier.
You might not have to wait that full decade to get mining going on, or to reap whatever other good resources are on this island, but it's still a lot of work for something that isn't going to make immediate returns on investment.
Each country may have already tried a colony before, only to run into too many problems for it to become self sufficient. Maybe it's happened multiple times for each country. Too many raids to defend against, too many monsters, malaria, bad village managers, scores of wolves eating too many livestock, poison ivy, and again the list goes on.
Besides, you might be fighting a war, or 2, with other countries, so colonizing even a wealthy land isn't your top priority. Besides, all that planning and stuff is boring compared to war.
Not actually uninhabited...
So, there's this guy... ok, lots of guys that got together and somehow survived this island after being hunted for crimes in their own countries. They are outlaws, maybe even pirates, that want the island basically for themselves. Sure, it's completely surrounded by enemy countries, but these same countries are where the
loot food, clothing, etc. comes from.
These guys and gals don't like it when one of these "self righteous" countries tries to "take over" their land, so they defend their
turf lair hide-away adopted new land by raiding the colonies, killing everyone or co-opting them into the gang family.
From the colonists standpoint, these outlaws are worse than the rival countries when it comes to raids. The outlaws use all the island has to offer, when it comes to attacking. Stampedes, attracting huge swarms of biting insects, tame wolves, you name it: the outlaws use it as a precursor to a raid. While everyone is distracted by the "natural" problem, the outlaws sneak around to the least protected spot in the defenses and cause major damage or even take over the village.
Frequent Natural Disasters
Massive flooding, earthquakes, sink holes, fire swamps, very active volcanoes, quicksand, and torrential rain all seem to converge in this island of easy to get to metals, gems, etc. Except that the metals are liquid pools, surrounded by lava. The gems are constantly churning in large lakes of mire. The grains are completely infested with biting insects that cause horrible, even deadly reactions.
"Sure, it's a great place for a vacation, but who would want to live there?"