Release a new dedicated handheld competitor into the market to chip away at the already-shrinking 3DS and Vita market?
No one at Microsoft who wants to make money is going to do that.
PS Vita is like, proof that a powerful handheld game device is just not enough anymore. You have to have amazing games, and not just ports or portable versions of console series with watered-down graphics.
The only reason 3DS isn't doing just as bad as Vita in the current climate is because Nintendo dropped price by a third less than six months after release, which is totally unheard of, and they've been releasing a steady stream of AAA sequels, to the detriment of their ability to release enough Wii U games.