Because the Vita was less popular everywhere outside of Japan, which makes that there are less units available and the units cost a few times the price of a psp second hand. (in The Netherlands the PSP is just under 100 Euro second hand, usually with 20+ game discs, a vita starts at 200 without any games and often without a charger)

A lot of people also still have a PSP just hanging around, less so with the vita.