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. 

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