Because Backbreaker uses real physics and the hits are momentum-based. The faster you are running at a player prior to a tackle, the harder his body will react to the hit. And the contortions and body movements react to actual collision physics between the player models - which means the chances of seeing the exact same tackle multiple times are very slim.
In Madden and in 2K5 games, tackles are all predetermined animations with random results based on the game engine's parameters. The tackles are almost all motion captured and then tweaked on a PC before the game even loads - that's why you see a lot of the same tackles over and over again and the players fall down the same way over and over again.