The fact that Nintendo owns Monolith Soft isn't exactly a compelling reason for why it should receive further content. It certainly makes it feasible, yes, but just being owned by Nintendo (as is the case for every other series represented in Smash aside from Sonic, Mega Man, Pac-Man, and Meal Gear) doesn't mean that it should get massive amounts of representation.
Furthermore, being a big hit does warrant representation, yes, but again, that doesn't necessarily mean it should get too much of it. All things considered, Xenoblade is currently represented in Smash by a brand new, fully unique character, a new stage, a handful of music and trophies, and an Assist Trophy. It's a complete package; not every represented series can even claim to have that. Is any more truly necessary?