Not gritty/mature(?) enough. Just one of many assumptions.
I personally like Eastman and Laird's designs, and feel they would fit well, even today (which this game's art gives me that feel).
One thing I did not get with the Bay movie (finally watched it for the first time two nights ago since this thread pop'ed up), is why Raphael was so damned jacked. I liked that one of them was, but he was always known as the "speed guy", Donatello, while the tech one they got right, was the slower of the bunch. He could have been a jacked nerd. But I guess writers always have to have "their thing".
Other than that, I was actually shocked that I did not mind the movie too much as far as effects went. Maybe it is because I am getting older and less nit picky. Though I did loathe the first "alien" idea, and they did poke fun at that in the movie. Nice little easter egg.