Because of its strategic importance to Russia (and Europe) which is the actual reason Putin invaded. The question is would he have taken Crimea (Sevastopol) or the Donbas or invaded Ukraine if it was a NATO member? He hasn't invaded other NATO countries. Inviting Ukraine into NATO was obviously risky. The time to do it was probably when they still had nukes themselves, but I don't remember all the issues at play back then.

But all this is moot. We are where we are. What should the US, NATO and the EU do now? Asking all this historical stuff is just a weak attempt to tacitly justify the unjustifiable of Russia invading Ukraine.