It doesn't make any sense at this point for any developer outside of Japan to sink resources into making Vita exclusives for retail release (MSRP $39.99 games)
Now I realize the thread is about a PS4 exclusive GR2 rather than a PS4/PSV Crossplay type of release, but the reality is that the market outside of Japan for the PSV is so small that developers need more than a "we have an obligation to those who backed a losing horse" motivation to justify such a release.
In Japan it's an easier sell since there is an actual market for PSV games, but the rest of the world has spoken and voted with their purchasing power; developers don't have to do any favors by potentially losing money on a product that the small PSV market won't even support.
If they made a GR2 port after the PS4 release, I'd consider that to be a big "we support the original audience for Gravity Rush even though it won't do anything to improve the franchise as a commercial product for the future."