I'm not sure there's such a thing as a "main" or "side" launch title in the sense that they will all be vying for attention at the same time. I suppose you could argue just how much overlap there is between MK and Metroid players, but MKW would have overshadowed Prime 4 if they launched together.

I doubt Nintendo wants to risk having Mario Kart cannibalize another one of its legacy series, especially given that Metroid can struggle having multi-million selling titles even under the best of circumstances. Still, this is just my two cents on the situation. I have no idea in actuality. 😛