TMNT is still not anywhere near the level of DC and Marvel superheroes in popularity, there is no guarantee TMNT will get constantly relaunched like Batman, Superman, Spiderman and X-men are.
I remember when this show started way back in 2003, people were surprised that TMNT was even getting a second shot. Of course most of the original series fans never bothered to watch this show, and the new young kids who watched it simply weren't mass in number to make it a hit.
In the future a company will say, "Well how did the last TMNT cartoon do in the ratings?" and they'll look at the dismal ratings on 4kids and pass on it.
Its sad but what can you do. Ninja Turtles will never be what it was in the 80's and 90's for those of us who grew up with it back then. The younger generation truly missed out on a phenomenon, and now we're seeing it fade away again.