Where are you even getting Sony has any inclination to do that, Jim Ryan has shown zero interest in backwards compatibility beyond what they have now.
Sony are working on PS3 emulation. Might not be too long before we can play PS3 games on ps5 after all.
Ah good point, I didn't spot that they were streaming PSP games too, I thought you meant games on the Vita itself. I assume then it's because the PSP controls can be easily emulated on a controller whereas perhaps the Vita can't, given it had some additional input options I think.