Gravity Rush 1 wasn't a big seller on the Vita (43k at launch in Japan compared to 6 digit figures for stuff like P4G and Freedom Wars). Not only that, but you could tell the Vita was struggling to play the original game what with the sub-native resolution & the long load times for challenges. With the original teaser video not specifying a platform, it was pretty obvious that Gravity Rush 2 wasn't happening on Vita - the sales weren't there and the technology wasn't there either.
Kinda neat Sony is giving it a second shot. I guess user ratings and reviews were high enough to reinvest in it.