Monster Hunter as a series is one that does best on portables. While it has had main instalments on home consoles before, they've seldom done well and never as well as the portable instalments. Due to 3DS and now Switch being the only portable devices on the market, it makes sense.
It's not Nintendo platforms that are enticing to Capcom. It's portability.
Plus they probably are going for a new engine, but are waiting for the audience to build and are bringing the audience over with this