Few worthwhile exclusives.
Poor 3rd party support.
Non-Gaming features were made irrelevant by smartphones.
Finally I'd like to add one thing that definitely did not kill the Vita. Smartphones being able to play games. People like to talk about how smartphones offered a cheaper ubiquitous gaming option, as if that hasn't always been a thing in the handheld market. Tiger Electronics didn't kill off the Gameboy, but somehow we blame smartphone games for killing Vita and 3DS?