Because it pushes the local ecosystem out of balance.
Think about plant growth. In general, things that promote growth are good. We get thriving forests, plains, deserts, seascapes. We have an explosion of wildlife, clean air, fresh water, everything a planet could want (even if it gets in the way of certain humans).
But one of the worst things for the environment is ... plant growth.
In real life, plants get nitrogen from the air, minerals from the soil, water as needed, and it's all kept in perfect balance from microbial life, earthworms and other animals in the soil, pollinators, and more. New plant and animal material (especially manure) rots to provide fertilizer.
In our all too real but artificially imbalanced world, plant growth is seen as science and only 3 primary elements: nitrogen, phosphorus, and potassium are considered important (plus a few minor ones). Application of these can cause all sorts of problems on farms (and anywhere the runoff goes, anywhere the water table touches).
What many don't realize is this happens unintentionally as well, far away from farms or gardens. Excess phosphorus—mainly from laundry and cleaning products—runs off into streams and water ways, it goes down drains into lakes, rivers, and the ocean. And it makes plants grow.
Phosphorus is one of the primary nutrients (along with potassium and
nitrogen) for plant growth. When phosphates were used extensively in
laundry detergents, the waste water carried the phosphorus to rivers,
lakes, streams and ponds. This led to massive algae blooms--a
condition known as "eutrophication." The algae depleted the water of
oxygen, which resulted in the deaths of large numbers of fish and
other organisms. (ref)
In your world, naturally occurring orgone sometimes fails to breakdown properly and it forms miasma. You don't say why this happens. Is it just random? Is it happening more often because of overpopulation or more use of magic? Or is the failure to breakdown happening in larger percentages because of something else humans are doing?
Regardless of the underlying mechanisms, miasma is like phosphorus, it spurs overgrowth and imbalance. Or perhaps you can compare it to mania. Healthy human minds are balanced but someone with bipolar has too much depression and too much "up." You might think the "up" part is desirable (you have tons of energy, get lots of work down) but it's actually pretty horrible to live through (though somewhat additive for some people).
Healthy bodies, healthy minds, healthy ecosystems don't have those ups and downs. They have balance and carefully tuned systems. This means they can recover from things that life throws at them. But if you push something out of balance often enough, it will harm it. Humans can recover from injury, but some injuries are too grave, or too repetitive to come back from. They can recover from the death of loved ones, but people who survive genocide are never the same again. Ecosystems recover from fire, but not from the determined reshaping of land and water by farmers.
Miasma is something living things can deal with, in small doses. But when it's a larger dose, or a more prolonged smaller dose, it's detrimental.