Because we created NATO to contain Russia, a long time adversary. Because we were part of the agreement for Ukraine to surrender its nuclear weapons with the guarantee of its sovereignty being secured from Russian aggression. That's why.
Ukraine is not in NATO. We should not have been part of the nuclear surrender. It was a European matter, and it should have been resolved by the Europeans. If the Europeans want to have a big economy that rivals that of the USA, then they had better figure out how to defend it, rather than expanding it at our expense.
There is no treaty guaranteeing Ukranian sovereignty, so any such arrangement is subject to the whims of the sitting POTUS. Thank God we have one now who understands the true price of war, even as the previous administration promoted bloodshed for its money-laundering operation..
Given the history and demographics of Ukraine, Russia has a far more compelling case for having at least parts of it in its sphere of influence than the Europeans have.
Ultimately, this war will be settled under terms that were available three years ago, and the country will be in ruins. That outcome cannot be prevented with more war. Trump sees that even if the neocons and global socialists do not.