I don't really know why, but I've totally lost interest in portable gaming these days. Maybe it's because I'm spoiled by my giant TV screen or that my iPhone has taken over as an ersatz Game Boy (with amazing graphics, see Infinity Blade 2). I also don't really have any friends who have handheld devices to play against (as opposed to my 360, where I'm always talking to one friend or another). Anyway, even though the Vita seems like a great machine, I can't really see myself buying one anytime soon.
It wouldn't surprise me if sales for both the 3DS and the Vita are slow, because I think I'm not the only one with these views.