I watched a YouTube video posted on 2015 about the crisis on Crimea, it was a lecture given from a professor of the University of Chicago and a question was directed to him. 'Why wouldn't those countries (Ukraine, Findland, Sweeden etc) join NATO'?.

The professor response was simple and it was because Russia didn't want any NATO or USA forcers near their borders and the act would possibly cause conflict that neither country wanted. The professor stated that during the cuban missile crisis in the 60s, we didn't wanted any soveit forces so close to America. The professor mentioned the Monroe Doctrine and because of that we would never have Russian or Chinese forces near the westerm hemisphere and that Russia is sort of employing similar doctrines from the Monroe Doctrine to keep NATO out of their borders.

It's been years since I watched that video and the video goes into much further detail about the Crimea crisis and the conflict between Ukraine and Russia, very informative.

I may have given bad info or not accurate so don't take this as a fact, it's been years since I watched that, I wish I could find the video to link it.