This is now becoming a crossover of the handheld forum but I still find it surprising that the Vita would have a higher software adoption rate than the 3DS. If I don't include the free titles given early on and by the Ambassador program and just strictly any game I've bought with my own money, I have 8 retail titles and 8 digitally downloaded games. And yet there's a bunch more I could easily buy but have been putting off to finish up what I already own and keep up to date. (Over halfway there! Yay me!).
To me, I just think that would be a similar case with most 3DS owners but maybe not. I suppose it could be demographics. If parents are buying it for their kids, they don't have the income to be purchasing many games and usually if I see anyone playing a PSP (whatever version), they are late teens or young adult and chances are, they have more income available and ability to buy more games.