The reason football players are so big, so fast, so explosive, etc is because of the cardiovascular requirements of the game. Football is not aerobic sport!!!!!! You can't play soccer at 6'7 340lbs!!!!! Follow along!
The reason Carl Lewis ran the 100, 200, and long jump and not the 100 and 3200 is due his genetic predisposition to running very fast. If he were to cross train with long run's, his 100 meter time would suffer. The reason sprinters take full rest intervals in between reps, is to to ensure they're fully recovered to maximize effort on the next attempt with the hope of maintaining velocity and power input. The premise is the same in football.
Rugby has similar elements as football but is played by slower and smaller athletes.
A good analogy would be comparing top fuel drag cars participating in demolition derby to rally or sprint car races.