Sony considers the Vita to be a legacy device, meaning they won't be supporting the system with 1st party games anymore. They announced this quite a while ago, so it shouldn't come as a surprise that Gravity Rush 2 is being released as a PS4 game. Developing the game for both a vita and PS4 release would likely hinder the PS4 verison of the game, and limit what they could do in terms of world-scale, or even gameplay.
Lots of people missed out on playing the original Gravity Rush because they weren't interested in the purchasing the Vita. If the first Gravity Rush wasn't convincing people to buy a Vita, then a sequel wouldn't do so either. With the series coming to PS4, I think many more people will get to experience the series, and the series will be better off for it.