Simply because it's more sure of what it wants to be. The 3DS is a gaming device, first and foremost. Yes, it has other features and apps, but it's primarily a gaming device. The PS Vita wants to be a multimedia device (just like the PSP), where games are important but not the full focus of it.
With iPhones and Smartphones, which have everything but big-budget games on them, the Vita simply can't compete and they're not going to buy another all-in-one device when they already have one. Nintendo knew the casual audience was fickle, but is more sure of who they are targeting with their system.