I don't think there is anyone in the world that could say with a straight face that this has anything to do with any kind of tech limitation, or with the manhours ($$) it would take to accomplish. (We all know the system has a PSP emu, I'm sure the PSone emu could run inside it and still be full speed)
So I think we all know what the real deal is here. Square (and other companies, and even some of the software arms of SONY intself) are all of a sudden realizing that they are not going to be able to constantly port their back catalog with premium prices. They are realizing that as soon as PSONE goes live on Vita, I have 20 new games to add to my new system, that I am not going to have to pay an additional dime for.
I think that Sony has realized that giving you rights to these games for all of their platforms forever may have been a mistake, but were they to make me rebuy the licences at this point, for my Vita, I think they realize that they would have a major, major, major PR issue on their hands.
Also, the lack of certain PSP games that work on Vita, (Valk Chronicles, etc) but aren't available in the store is curious and I can't really figure it out. I guess SEGA really really doesn't want people who have already paid for it on PSP downloading it again on their VITA for free. (I'm aware there is a pain in the ass workaround that requires a PS3, but, c'mon)
ALL OF THIS IS CONJECTURE and opinion. Please, educate me if necessarily.