Many reasons
Highest Userbase
Lowest Dev costs
the success of M&S and the O G - Sega definitely want to turn M/S into a franchise, developed by them but with help (as happened for the first) from Miyamoto, and to do that they want Nintend happy with them
Sony did to Sega as MS is currently doing, which nearly bankrupted Sega and drove them out of the hardware business. Whist companies aren't petty enough to be completely ruled by things like that, I'm sure it had some influence







