1954 USSR proposed to join NATO and got rejected. 1991 one of Yeltsin's long term goals was to join NATO. 2001 Putin asked Bill Clinton if Russia could join NATO and his response was "never mind". So why was USSR/Russia never permitted to join NATO? What would be downsides of them joining NATO?
This is really a question? NATO came into existence BECAUSE of the USSR. Since the USSR was seen as an aggressor in the post-WW2, bases on their behavior in Eastern Europe, some Western European and North American countries saw the need for a defense pact. The USSR applying for membership in 1954 is nonsensical as by then they had created the Warsaw pact (1951) as a reaction to that same NATO. This is basic history stuff - every schoolkid gets this in highschool.