The fan base is locked at around 4 million to 5 million, and it migrates from one primary Monster Hunter handheld to another, it seems. If they put one on Vita, the sales would likely be equal to or less than Monster Hunter 3G sales on 3DS as opposed to Monster Hunter 4 sales. Why do that when that same game can be on 3DS, re-use existing assets, and sell for 4 million or more?
Two factors:
1.) You're describing a shift, not a multiplatform release. PSP was on the way out, and Capcom chose the 3DS to carry the franchise forward, most likely because they could re-use many assets to cut costs.
2.) The fan base is locked. How many people are there in Japan that don't own a 3DS but are fans of Monster Hunter? Unless it's a sizable number, there's no reason to do it.