I can be either very right on this or very wrong. But Nintendo developers have said they really want to make a new 3d Mario on the DS.
Now, they have no business making a 3d Mario no the DS. The DS is awful for controlling a character in 3d. The graphics on the DS are not fun for 3d. Also, if any game warrants another Mario game on the DS it would be a sequel to NSMB DS. After all, a game that sells 20 million plus would certainly be crying for a sequel.
But no, we we will get a new 3d Mario DS. The reason why is because Nintendo developers like to make games for themselves. They had to be dragged kicking and screaming to make a 2d Mario.
It took twenty years to make a new 2d Mario. Yet, they are popping out 3d Marios like candy. Clearly, you can tell which type of game Nintendo developers prefer to make (and also reveals the gap between Nintendo and their consumers).