I think this fits mii costumes as well, and I understand it too; wanting to fit in as many characters as they can even if they won’t be playable, prioritizing other characters as fighters for various reasons while still wanting to let people see their favorite characters represented in the game in some fashion. A very good reason; they can’t make everyone playable, of course.My two cents on the AT issue is this; the staffers may have wanted as many characters to directly participate in the battle as possible without concerning themselves about reworking them as playable in the same game, so that they can prioritize other things. If that's indeed the case, I can understand, even if they won't confirm it.
We probably will never hear why the likes of Waluigi, Skull Kid and Ashley keep being left out, so all that can be done is trying to rationalize the reason for that.
I still think it leaves the possibility open in DLC if they happened to reconsider a character to be a fighter, like they did Rex before thinking their idea would be too complicated. If not, it’s all good and understandable of course, they just happened to prioritize other characters for varying reasons, doesn’t mean they stopped caring about a character because “we made them an AT already, we can’t do anything with them now,” as people seem to imply when insisting ATs disconfirm (and as they seemed to when insisting spirits and mii costumes disconfirmed). I’m sure Sakurai would’ve said “we already had them in the game in some fashion” if the AT or costume themselves were really why Spring Man and Rex weren’t chosen (with clear reasons being given as to why not them anyway, them even seeming to try Rex).