Several NATO powers, most notably Germany, have been standing in the way of eastward NATO expansion for some time now. Additionally, Hungary, a NATO member state, has an autocratic prime minister in Viktor Orbán who is presumably not all that averse to at least some of Putin's tactics. The votes aren't there, not that it's an overnight process in the first place.
The EU is somewhat hamstrung as well, given how much of their energy they buy from Russia. So the issue is fraught on quite a few levels, as Putin well knows, and the answer is nowhere near as simple as "show leadership."