Because they are run by right-wing oligarchies that want to consolidate and protect their accumulated wealth and power? The imperialism is coming from inside the house.

Russia after Yeltsin

Russia during Yeltsin rolled in the tanks on its own parliament. The absence of foreign invasions was not for lack of malice, but for lack of capability.

The reason why ex-Warsaw Pact countries are flocking to NATO is because when the communists left power, the reactionaries resurged. And naturally the reactionaries in power wanted to be part of a right-wing alliance. But no matter what revanchists might tell you, living standards across Eastern Europe were better in the 1980s than they were in the 2000s.

Because the US starts color revolutions in those countries until a pro-western government is in power.

That's a good question. Let me tackle it from a different angle though - why do ex USSR/Warsaw Pact countries actively want to join NATO?

Fellow ex Warsaw Pact resident here.

They wanted to join NATO because after the dissolution of the USSR these countries were pushed into a deep economic crisis, to which one of the solutions, apart from relentless austerity programs was the privatization of the shit ton of public assets they had. Of course lots of western companies were in on this since for them these assets were really cheap and they had a lot of money. The city hall of the town i went to university to became a fucking McDonald's.

Thing is, a lot of people didnt like this, not just the austerity, but the handing of domestic assets to western companies. And they were not even that wrong about it! In Albania, in 1997 a series of bankruptcies of asset managing companies (most western owned) who were basically scamming people who barely came into contact with capitalism, telling them theyll get 50% interest rates for their money, led to a brutal uprising where ordinary people were sacking military bases, setting up machine gun nests in the borders of cities and overthrew the government (after half a year of protests).

In the meantime Russia was led by well-known alcoholic, Boris Yeltsin, who doesn't strike me as the napoleonic conqueror people make him out to be.

So why did these countries join NATO? Because they DESPERATELY needed the money, but western companies wouldnt invest in (exploit) them if they dont have insurances (troops that could be sent against the people anytime an Albanian-type revolt breaks out or an anti-western government come in power who would try to renationalize assets) that their investments (exploitation) runs as smoothly as possible. And it works. People like to say that "ackshually the living standards went up in Eastern Europe", but they never stop to check that it only went up because the rich got richer, pulling the average up. The working class' lives stagnated at best, except the social net around them is rapidly brought down. Older people are not nostalgic for socialism here because theyre becoming senile, but because they see every time that they go to a hospital that the increasingly privatized healthcare system is crumbling.

Don't believe me? It's fine. But i would suggest that you examine who the current pariahs are in NATO: Hungary, whose government has to rely in a lot of things to the cheapest due to a ravaged economy (both by corruption and privatization), so they rely a lot on domestic production and trying to hand off as little stuff to western corporations as possible (and still fail at it, hence why they are still intact), and Turkey, who makes no secret of wanting to standing on its own feet and not rely on western corporations.