It's as simple as that. Floyd Mayweather might be the best boxer pound for pound, but he is simply not the best ever because he isn't fighting in the heavyweight division. There are currently around 100 boxers at minimum who could beat Mayweather right now. Tyson Fury would murder him in the first round. A boxer who isn't a heavyweight cannot be called the best. He might be the best boxer ever in his weight class, but that does not matter for the majority of the boxing fans. Ali was also called the greatest because he wouldn't lose against any boxer in his prime. Mayweather can only become the greatest if he moves up to the heavyweight division and beat Klitschko.
