PS3 sales aren't quite good during their first few years either, mostly because they have crappy initial game libraries, and the price, which makes people stick to their PS2 or purchasing the earlier X360 instead
the same thing can also be said with VITA, i mean about 80~90% of its game are already on PS3 and other consoles with only a sliver of them a VITA exclusive, if you already have a PS3 or a 360 why bother purchasing an expensive handheld to play the same game? also, PSP is still highly popular in japan, with games still coming out for them at least until next year.
this is the difference between VITA and 3DS, 3DS goes boom during its debut with 70% of their game not available on any other gaming platforms, with promises of even more exclusive games coming soon (like PW vs layton), that piques people's interest to get it early(only to find out that its region locked and some games are bound to be JP only forever)
but given time, VITA will have more signature games and Sony will stop making stuffs for PSP, then we'll see how things really going to stir...