The technical reason is obvious. App development occurs on top of a massively bloated and ineffecient UI toolkit where you can dev most of your UI in xml. I would be absolutely shocked if ps vita used anything like that. They're using opengl or similar and coding the UI is going to require more coding expertise but the performance is going to be great. You could do the same on a smartphone of course, and get maybe even better perf compared to a vita, but no one does (unless its a game).