If P5 ends up on the PS4 I guess they'd want to get a head start in development like they have for PS2, though they'd be ignoring the people who own PS3s already who are ready for a Persona game, and it's easily the safest choice. But it'd mean they don't have to wait a couple years to get something out for next gen like this one.
Theoretically they could reuse assets for games other than Persona on the PS4, but when SMT4 is on the 3DS you have to wonder what. I guess Catherine 2 is a lock since Catherine sold well and Index said they wanted to grow the brand, but what else? Another new IP (possibly still part of Megaten)? IDK.
They might have gotten an incentive from Sony to do it instead of sticking to current gen but tbh Sony doesn't really care for JRPGs outside of FF so it'd be weird/new for them, although Sony does help with Persona's media aspect.