The Vita is not a PSP. It just has a PSP software emulator. Software emulators are never perfect emulators of the actual hardware, so there is no certainty that all PSP games will run in a playable manner on the Vita. For that reason, each PSP game must be tested and certified playable on the Vita before it can be made available in the Vita Playstation Store, or installed on the Vita by the Content Manager. There is a fee for the certification process, and the publishers of many older PSP titles choose not to pay the fee, because they are not confident that the game will sell enough copies in the Playstation Store to jusify the investment. That is especially true if the game didn't sell a lot of copies when it was released at retail. Most of the PSP games in the Playstation Store will never be certified on the Vita. Except for new releases, we probably won't see many more than we have already.