From a technical standpoint, the Vita is a better overall system and it has better quality games. So why do people keep on buying the 3DS?
It might have some better quality games, but The 3DS has more quality games the the vita right now. Some people would rather have tons of great games than 2 or 3 awesome games. And even then, The 3DS does not have "better" games than the 3DS. They are all actually quite comparable. Just different experiences really.
Now Playing (VITA): Ragnarok Oddysey, DJ Max Technika Tune (soon) PSN: Roksor. Now Playing (PC): TalonRO
I'd say kids. Don't take this wrong way but i think 3ds is more appealing to people age 10-15. It doesn't mean there isn't adults owning 3DS too. Also 3DS has physical backwards compatibility with DS games which is something which hurts vita.