It's simply a matter of contrast, it's all very well comparing the game sales directly, but in comparison to how many potential customers can actually buy the software, what we see is the following.
3DS
Monster Hunter 4 - 3.23 - 1 out of 13 owners bought
Kingdom Hearts 3D: Dream Drop Distance - 1.28 - 1 out of 33 owners bought
Dragon Quest VII - 1.24 - 1 out of 34 owners bought
Puzzle & Dragons - 1.16 - 1 out of 36 owners bought
Mario & Sonic at the London 2012 Olympic Games - 1.07 - 1 out of 40 owners bought
Super Street Fighter IV: 3D Edition - 1.01 - 1 out of 42 owners bought
Dragon Quest Monsters: Terry's Wonderland 3D - 0.96 - 1 out of 44 owners bought
LEGO Star Wars III: The Clone Wars - 0.89 - 1 out of 48 owners bought
Sonic Generations - 0.83 - 1 out of 51 owners bought
Resident Evil: Revelations - 0.76 - 1 out of 56 owners bought
Professor Layton and the Legacy of Civilization A - 0.64 - 1 out of 66 owners bought
Variance - 1 in 13 to 1 in 66
Vita
Assassin's Creed III: Liberation - 0.89 - 1 out of 8 owners bought
Call of Duty Black Ops: Declassified - 0.87 - 1 out of 8 owners bought
Persona 4: The Golden - 0.60 - 1 out of 12 owners bought
Need for Speed: Most Wanted - A Criterion Game - 0.56 - 1 out of 13 owners bought
FIFA Soccer - 0.47 - 1 out of 16 owners bought
Mortal Kombat - 0.42 - 1 out of 17 owners bought
Rayman Origins - 0.40 - 1 out of 18 owners bought
God Eater 2 - 0.38 - 1 out of 19 owners bought
Ragnarok Odyssey - 0.32 - 1 out of 23 owners bought
FIFA Soccer 13 - 0.30 - 1 out of 24 owners bought
Variance - 1 in 8 to 1 in 24
As you can see, while sales are indeed higher across the board for 3DS, relative to actual hardware ownership, Vita owners have a much tighter grouping and a higher margin of owners buying each piece of software.
So, the question you ask is, why do third parties support the Vita?, because vita owners buy games, and comparatively, a third party title on the vita is bought at a higher ratio too.
Not in higher numbers of course, because theres an obvious and large difference in overall install bases, that said, based on the install base it has, the sales performance versus install base is very similar (actually, slightly higher) than the 3DS.
Sorry to piss on your parade.