because back then the game was more competitive, teams wanted (and tried) to beat United, they had to really get out there and perform if they were to reach the kind of success they eventually achieved.
These days the gap between the top and the middle of the league means that teams don't go to places like Old Trafford to compete. They just set themselves up to try and withstand the pressure and hopefully grind out a draw. They even rest players they know will be able to get them points elsewhere to save them from the risk of injury.
United don't need to play as well as they used to because they don't have to. The opposition will lie back and eventually United will pop one in from somewhere.
So said Paul Parker in yesterdays Guardian, and I can see where he's coming from.