1. Because its never happened. Just like Third Parties, DLC, Guns, Sexualized characters, and many other things at one point. People like to make up rules and pass them as set in stone when really the rules are......"more like guidelines" to quote a damn good pirate. I want a Cross-series Echo fighter just to laugh when yet another "rule" is broken this go around.
2. "Because then they have stuff on the disc that they would have to disable," even though we can have 8 Ridley's already. "Oh no there are two of the same skin now and my terrible eyes can't tell the difference between a CPU and myself!" So, we can disable (well items in general cause they are obnoxious a day after release
) that specific trophy if the character is selected. Boom. Easy-peasy problem solved. Despite the easy solutions people are gonna pretend that this is still a major issue, for some reason.
I know it would be a nonissue, but even I don't think they'll do it. When I think DLC I'm thinking about big names. If I were Sakurai and I got to choose between looking for a "Cloud" or a "Waluigi" (who I already passed up this game) I would 100% look for a new "Cloud." Then again, maybe the fan outcry would steer Sakurai the other way? I dunno.
I will say its a really bad idea to think this is something that could happen. If you still have hope for Waluigi or Ashley this game you should probably put it out. If Sakurai does some crazy **** and upgrades them for DLC, I will actually cry man tears. Realistically its just not gonna happen though.