Is not that they can't. The problem is that they don't want to shoot themselved on the foot.
Sony is placing a huge investment on 3D television, and the PS3 is a good way to push the sales of those devices that are very expensive to make. If they make a 3D psp, they run teh risk of killing what they want to push and placed a lot of money in.
I don't think the next PSP will be 3D, but will offer a lor more features that the PSP and the 3DS







