Since most other people have offered nice sensible answers (smaller dev support; lots of cross-platform development due to ease of porting; cross-gen stuff) I'll just address all the examples that you listed in the OP:
| Fusioncode said: Toukiden |
Sony assisted in development & advertised in Japan.
| Fusioncode said: God Eater |
Was already in development for PSP, easier to port to Vita as a multi-plat than start development again or something.
| Fusioncode said: Demon Gaze |
Released in January 2013. Was probably already in development pre-Vita launched.
I'd point out that this game sold well beyond expectations too, which probably led to Experience Inc. committing to the platform for many more games (and Kadokawa too)
| Fusioncode said: Hatsune Miku |
Already in development before Vita launch. Hasn't sold any better on 3DS or Vita so not like there's any alternatives.
| Fusioncode said: Persona 4 |
In development before Vita launch. DAN is probably a reaciton to very strong sales of Persona 4 Golden.
