Because while Russia was in a way stronger position than western media was portraying, they don't have an infinite capacity for pain.

They would absolutely love to not be sending their kids to die and selling natural resources to whomever again and traveling around the globe without sanctions.

The concessions they would have accepted at the beginning were less. However, now that they've lost so many men and risked their regime on it, they can't simply accept the same deal.

This is why the incentives that wars create are so terrible, especially for authoritarian regimes, even if they're at fault.