Because its a used handheld out of circulation., with 2 large libraries(3DS and DS), plus GBC/GBA/GB, and the billion eSHOP ports of arcade classics.
Also the 3D gimmick on the N3DS models are really really nice, especially since nobody has tried to oneup them yet.

Why would you even buy a AYN over anything that could just be in the same form factor as the PSP?