I feel like they'd put in a more established character for their first rep of a new IP then bank on someone who people don't know and are unfamiliar with. This is ARMs first true inclusion into the Smash roster, and I feel they'd want someone with either the status (like Spring Man or Ribbon Girl) or popularity (like Min Min) to sell it since they have put themselves into the minds of those fans already and would bring those fans into Smash just so that they can play them. Someone new wouldn't have that under their belt, and they'd have to do more to sell themselves (and by proxy, their franchise) to establish themselves as a character worth buying and playing as, which may not work out depending on who they are. It'd be riskier to pull off, and while they could do it I feel they'd opt for a safer option since at least they know that the older characters have their appeal.