I wonder if they're worried a level editor will make it hard for them to easily milk the series again. Then again, it would simply require Nintendo to do more than throw in a couple new powerups, make new levels, and call it a game. Coin Rush, Boost Rush, and Challenge levels are steps in the right direction. They add some new concepts to a tired series. But they are not full games, just tacked on, and some people aren't even trying them because they aren't the main game.

It's unlikely Nintendo will drastically change the formula as long as the previous titles are selling. This is one of Nintendo's safest forms of income. The innovation they try on other franchises result in massive fluctuations of sales. The NSMB series is easy money and I understand why they're hesitant to mess with it, they need some lower risk releases.

All in all, a full level editor would be great, but Nintendo adds so little new ideas per title that I think they may see such a feature as damaging to their future sales. They would require to innovate one of the very few series that they don't want to mess with. It's like finding a vein of gold; it's better for them to mine as much gold out of there as they can before they start spending money looking for other veins.