Because Female Robin tends to be more popular/well-received than Male Robin. I mean, Female Corrin is considered to canon while Male Corrin isn’t by fandom, and Female Byleth is widely more liked than Male Byleth. They probably looked at the numbers and stats, saw people playing as female Robin, and decided to go with her since they wanted Corrin in their Dragon form.
Unfortunately that's a dependency between Smash and the actual FE fanbase that prefers Male Robin. Either way, the others went with default designs, as far as we can tell, so, really, they should have used Male Robin if just for consistency; don't see them using a different Villager. Good grief, they already made her a face of the FFA tourney (and the tips as well as the only designated Robin tile on the challenge board), but this too? They did at least give him the...electric themed one? But still, as far as official stuff goes, they should stick with defaults. Not to mention that FFA banner.
Also, I think the Byleths are more split...except in Smash where of course they go for her. But even then, you can't really prove one is more popular than the other. Sure, the YouTubers may prefer the females, but they are an incredibly small portion. It seems to be perpetuated by people seeing them more so they assume they are more popular, but being used by more notable people doesn't necessarily mean more popular. Given how the polls usually go, there is certainly a case for Male Robin's popularity.
Also, I'm not even sure if they bother to track stuff like that. The game doesn't at least. It cannot tell me whether I use Olimar or Alph more.
