I have mentioned that before, but I believe some of the reasons for that are:
a) Portability and Individuality - an independent system that you don't have to plug into anything else and that can be played discreetly anywhere.
b) Price - much cheaper than a next-gen console.
c) Dual screen featuring a touch screen - more screen space for instructions and buttons that can be accessed using the stylus equals less need for memorized control schemes.
Am I forgetting anything?