Because realistically they are more recognizable as Mario characters than anything SE has. That's likely more the reason SE has done nothing with the SMRPG characters in FF, DQ, etc. in the past 20 or so years. They may own them but it's not really a part of their "brand". Which makes it all the more curious as to why NoA is this year referencing SMRPG/Geno more than they ever have. Like
Fatmanonice
indicated huge companies like Nintendo don't post things things on social media or promote specific things without huge oversight. I briefly dated a girl that was in charge of the Starbucks twitter account and she told me stories about how much of a huge operation full of stress, red tape, and approvals from higher ups posting the most innocuous of tweets was. So I don't for a minute buy the idea that Nintendo referencing Geno/SMRPG as they have to be a random coincidence.