Because the 3DS is priced to be able to compete with tablets, and also has its own system-selling software. Parents see a 3DS with Mario, or Pokemon as a better alternative for kids than a tablet with doodle jump and temple run (though I'm sure many parents are still just letting their kids use their tablets while they're out running errands).
PS Vita, on the other hand, is more expensive than a good android tablet (as well as the 3DS), and also has barely anything on it that people find value in.
If Sony were to make it a goal to get an exclusive, system-selling game for the PS Vita then they'd have no problems, but they aren't really trying to do that. They're just getting indie titles on it, and a few niche games. Anytime they do put some effort into making a game for it it's typically a stripped down version of a console franchise.
Sony needs their own Pokemon, and they probably shouldn't rely on 3rd parties to make it for them.
Vita is only $30 more than 3DS now. I really wish that the vita was doing as well as the 3DS, it's so much better than it in terms of hardware.