I'm going to hazzard a guess & say it's because it's not compatible.
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...but its compatible on the psp? makes no sense to me
Yeah, they have to pay for the certification and testing process, and like I said before, they don't believe its money well spent, unfortunately, because they'd be great to have on the Vita.
You can play ALL PSOne titles via remote play, including those in the Crash and Spyro series.
True, but you can't really play them too far from your PS3, so it defeats the purpose of "gaming on the go".