I wrote a feature on de-extinction of aurochs for the Washington Post a few years back and had the opportunity to talk to a lot of experts and read up on the history of the aurochs, so I think I can help you out here.
The aurochs was probably more a creature of open plains and mosaic landscapes of plains mixed with scrubby and forested patches. The old growth forests that the species is described as living in for the last thousand years or so of its history were not a preferred habitat -- these were just the only places left where they could exist at all.
The remnant population in Poland was surviving on food and in habitat that was probably less than ideal. More importantly, they became genetically isolated and seriously inbred. We don't have specific data about the genes of very many of those animals, nor do we have reports from biologists on their health. But we do know what usually happens to mammals that face a prolonged genetic bottleneck like that. Look at the blackfooted ferret for a modern example. Sperm motility is low, many males are cryptorchids, which is to say that they are missing one or both testicles. Natural reproduction drops.
Population dynamics are a numbers game in the long run. Even if you have a group of animals that look ok, if their rate of reproduction drops so low that they can't keep pace with adult animals dying of old age then eventually they are going to disappear.
Here's the piece I wrote on the subject:
You may be interested in this answer from 3 years ago: https://old.reddit.com/r/AskHistorians/comments/cbh2be/in_1627_the_last_aurochs_or_wild_cow_died_in_the/etgduya/