In my case, having skipped out on the PS3/Xbox360/Wii generation of consoles(do to my current living conditions, consoles would be a pain in the ass to maintain), every game on the Vita, minus P4G, was something I'd never even heard of. A good number of the games interest me, as well as a number of confirmed and possible localizations(and, like, 3 indies). The other handheld on the market, the 3DS, doesn't reach my minimum of 10 games I would buy to get it. And as a PSP owner who loves buying digitally, the Vita's ability to play PSP games was a bonus. These four reasons made buying the Vita a no brained, but of course since I'm a cheap bastard I held out 'till my PSP started crapping out. Lucky me, it didn't get serious until I'd gotten a job.
tl;dr: Didn't own a PS3/360/Wii, 3DS doesn't have enough games that interest me, Vita has numerous games and upcoming titles that do, BC with digital PSP games is sexy.
3DS gives me horrible hand cramps after 10-15 minutes play-time even with Circle Pad Pro. Vita doesn't even if I don't use my Nyko Power Grip.
Not much of a fan of Nintendo's exclusives/first party titles. They can still get me with Metroid but they don't bring those out very often. Bayonetta 2 will be why I get a Wii U though. I tend to like Sony exclusives more and the Vita's (small) library has a handful of games that make the system very much worth it.
I work night shift and have a lot of downtime. I need a portable gaming system that isn't a phone.