The CGI and overall vfx.
They wanted the Turtles perfect to show off.
It's really the same reason we have barely gotten any promotion for 'Dawn of the Apes', and that film's titular characters, which comes out in 3 months, rather than 5 like TMNT.
But I'm sure there will be more heavy promoting for 'Dawn of the Planet of the Apes' in the next couple of weeks. And we got a solid trailer for the film a few months back which I'm sure many more highlighting the titular characters will follow. Plus, movie theaters have a poster up and some cardboard standees. What does this film have? Also, do we have any clue what the rating for the movie is? I checked IMDB, Comingsoon.net, and Wikipedia and they don't have anything.
As other people were saying, if you only show the Turtles hiding in the shadows or glimpses of their bandanas and weapons--- that won't get the general public excited. The public wants to see these guys moving, fighting, and being themselves.
Yes, people know the origin story and because of all the different adaptations and long history of their comic run, they have expectations of how they should look, sound, and be. With the changes made, while new fans and kids won't care so much, it's us older fans that are on the fence. You have some of you who just don't care, but others of us where it's difficult to accept the choices made.
This being all said, they are going to have to hit a right tone with the movie, explain the changes made, and present something that fans old and new alike can get behind and so far, with a lack of photos, poster, teaser, trailer, or interviews... they aren't really doing a good job with it.