All of the points mentioned below are valid. It is also often pointed out that their mission was to conquer and hold. That is quite different from our mission.

My personal observations are that they were refighting WWII and their limited use of modern air power prolonged the resistance. I also believe that the Russians are over rated as an offensive force. While we rightfully feared nuclear retaliation during the cold war, imo there was never a time we couldn't have kicked their butt in a conventional war fought anywhere but mother Russia.

Considering how the USSR ultimately collapsed we probably could have beaten them there but fighting on native soil is tough any time and the Russians are at their best in a defensive battle.