Because it’s not just any TLD. It’s one of the very few special ones that has a very clearly defined origination and governance (well it did for the first decade or so at least).
Capitalism + the underpinnings of the Internet don’t exactly play well together (hence the need for clearly defined net neutrality codified by law). And arbitrarily raising prices on already existing domains registered by people and non-profits who bought in when there were rules in place about such prices is actually wrong.
It's not like .org lives in a bubble. There are many other TLDs that they have to compete with on price.