The thing is - some mechanics are a bit different in others modes, namely Escape and Survivor, where the Marines are much tougher (they got Armor just like in SP), and the Aliens can't use one hit kill mechanics such as Fatalities. So in these modes, Aliens have a much harder time getting kills and need to work together and attack in swarms a lot more. Marines must also be downed first before Aliens can finish them off, and they can get revived quite quickly.
So it would make sense some players would rather play TDM and Extermination more, as player health is a bit more balanced there.