Oops, I meant, "Why hasn't the PS Vita surpassed the 3DS in sales yet"?
Sadly, I don't think it will ever happen(although I really hope it does).
Nintendo has had the top selling portable handheld since the original gameboy. Almost every generation, there was a more powerful handheld along side it, and Nintendo always comes out on top. The gamegear, lynx, neogeo pocket, turbo express, PSP, ect. These where all better systems hardware wise, but they all suffered from poor battery life, where Nintendo's product always exelled(until the 3DS).
Nintendo also has the greatest selling first party games in the history of video gaming. At the end of the day, Sony just doesn't have the games that have a huge following worldwide. Uncharted is a great western game, but not so great in the east. Mario Kart, Zelda, Mario platformers are all HUGE million plus sellers worldwide. Sadly, most of these games are just repeated every few years, but they sell like hotcakes no matter what.
This is the main reason I don't believe Nintendo will ever loose their top spot in the handheld market. The PSP is selling really well in Japan right now, and it is pretty much dead in every other region. So great software can change the tide at any time.
The Vita can turn itself around, but it will take a huge focus and level of comitment that I just don't believe Sony is capable of. Great games are required, along with dedication to deliver apps and promised functionality in a timely manner. PS1 support was promised, and there has been no mention of it since. The memory card pricing is another black mark on their record, and their content manager is just plain lame.
I really love my Vita(and I think it is superior to the 3DS in almost every possible way), and I've played it more in 2 months than I have played my 3DS in a year+, and with Gravity Rush and Resistance coming in a few weeks, I don't expect to put it down anytime soon. So I'm hoping for some killer announcements from E3, but I won't be suprised if Sony drops the ball there as well, as they just don't seem to have a clue.
Chris