Where did you get those Vita numbers???? 

3DS sells average 100k a week and Vita sells around 30k, Vita numbers are no where near close

I think the memory cards had some sort of anti-piracy tech in 'em hence the ridiculous prices. Modding happened a lot on PSP and I imagine games were pirated. Piracy is very very scary.

Yeah, I think it has a lot to do with it being marketed as a handheld "console experience." It had plenty of spin-offs of AAA console games within the launch window, like Uncharted, Assassin's Creed, Call of Duty, God of War, and Resistance, but Sony greatly misunderstood the handheld audience. You can't just move a AAA PS3 series to a handheld and expect people to keep buying it, especially if it's pretty much identical to the console game in style and gameplay. Nobody is going to play Call of Duty on their 5" screen with tiny thumbsticks when they can play it on a 50" screen with large thumbsticks instead.

I guess that all ties in with the lack of killer app though.

The memory card thing is also a huge issue. I can see it being a tactic to keep the price tag of the system itself low enough to be competitive with the 3DS, but most consumers aren't stupid enough to overlook the "hidden" cost of proprietary memory cards that are apparently made out of solid gold.

It's going to stay a niche device for JRPG and VN fans.

Why has Vita sold 5 times worse than 3DS though?

Because Sony relies on third parties and those (specially the western ones) have chosen to support mobiles instead of the Vita. 3DS still has Nintendo games.

Vita needs a pokemon game :/

The answer to a successful handheld is pretty easy.

1) Keep the hardware simple, reasonably powerful, economical, cheap, slim, with long battery.

2) Keep the development and purchase of the games easy and convenient. Download is preferred.

3) Keep the games simpler & cheaper than the traditional path, and open up to indie games. Handheld platforms are not meant to be console gaming on the go.

4) Do not use propriety hardware for the fundamental features such as the memory card.

http://www.thesixthaxis.com/2014/07/31/sony-romped-to-3-5-million-playstation-home-console-sales-last-quarter/

Which was talked about from Sony's financial statements.  I recall the numbers when they were both statements were released before but no one seemed to want to talk about them.  Which brings the other elephant in the room no one wants to talk about here, the 3DS is way overtracked recently.

No killer games such as GTA series means no sales & Memory card is expensive.

The hardware is more powerful than 3DS'.... Real embarrassing.

The reason of Vita downfall was it being marketed as portable AAA machine, and then no one bothered making those AAA games. Sony itself destroyed interest for 3d party by releasing Uncharted GA which was obviously far more ambitious than what 3d party was willing to do with Vita, setting expectation bar too high. Meanwhile Vita is not "pocket PS3" by any means and following the whims of engineers by letting them get proprietary card format didn't help either.

What they should have done instead is copying Nintendo approach of making portable version of home console 2 generations behind, i.e GBA was SNES portable, DS was N64 portable, new 3DS looks like Wii/GC portable. It instantly creates a decent library of ports while not putting real competition, since these games are not new really. Sony probably thought PS2 lacked the hard hitting exclusives and tried pushing the focus to PS3, but the hardware wasn't ready for it.