I can defend it. Here, lemme try.
Multiple characters were chosen out of pure circumstance: Roy to fill out the roster and promote his coming game due to convenient timing and lack of substantial effort, then Lucina due to not working as a Marth skin to Sakurai's liking and, again, filling out the roster.
Corrin was then chosen in much the same way Roy was: in order to promote a coming game because Nintendo saw a huge upswing in Fire Emblem in the west and wanted to capitalize on it. Sakurai wasn't even sure it was a good idea at first, as written in Famitsu (paraphrasing from the English article covering it):
Then Ultimate's whole pitch was "EVERYONE IS HERE". Everyone includes every single one, which bolsters FE in particular because that demands clones (Roy, Lucina) and unpopular picks (Corrin) to satisfy that promise. Not to mention Roy was added to Smash 4 due to fan demand, then Chrom to Ultimate, again, due to fan demand.
Let's stop pretending there are no FE fans asking for these characters. There are a lot of them.