Aye, I noticed that. The $25 million opening weekend was pretty good, all things considered, but for a mid-range kid-oriented animation like TMNT to have legs, it really needed a clear run at the kids market for a little while. In this case, Meet the Robinsons came out the very next week with its hefty Disney marketing clout, so that pretty much hoovered up all of TMNT's business from the second week on.
It's a shame, because retrospectively, I really enjoyed TMNT...it was a flawed film, but at its best it really nailed those characters and struck just the right tone. I for one would really like to see that creative team tackle a sequel, because if they kept what worked about the first one (the look, the mood, the action, the group dynamic), and fixed what didn't (the human character designs, the incoherent narrative, the weak villains), then I do think they could produce a more or less perfect Turtles film. Like Pawbah said, the film has made profit, but I just wonder if it made enough for them to regard a sequel as financially worthwhile. Here's hoping.