From a consumer stand point, unless you really want console gaming on the go, a smartphone/tablet is just way better value than a PSVita.
From a developer standpoint it's costs a fraction as much to develop for iOS/Android, or even 3DS, all of which have a userbase many times that of PSVita. Why spend the money to make a Vita game for 2m people when you could use that same money to make 10 iOS/Android game for what? 100m people?
A huge developer like Sony should have seen this shift coming (hell it was already here!) and adapted.