If it was M, it would allow them to do other things with the fatalities, but since it's T there certain things they can't do, like limb ripping, extreme deaths, so on and so forth.
If it was M there would be a much much larger amount of 'moves' or ideas they could use, and you can bet your sweet ass that they've thrown out dozens of fatalities because they either couldn't make them fit with the game mechanics (no limb ripping means no limb ripping, no ripping people in half, so cuttin gin half, so on and so forth), or DC wouldn't allow it.
Infact, Cinder(the DCvsMK board leader/worker of midway) said towards the begining of the hype for this game over on the Midway boards that they've had to tone down a lot of fatalitiy ideas and had to throw out even more due to the rating and DC's disapproval.
I mean yeah it would still have some of the campy overthetop blood, but still, I'd much rather see the character run around on flames and burn up than just showing the flame hit them and they turn to ash instantly, in the case of Scorpions Toasty.