Becuase that would mean that Nintendo took the time to go out to Bethesda, an american company, to negotiate DLC content for Smash, and either Nintendo or Bethesda decided that all they wanted/were willing to provide, was a Doomguy Costume and a Cacodemon Hat.
With regards to Nintendo, why would you go out of your way to negotiate with a foreign company and NOT want to get a playable character out of them? Sure, they've done that with Wayforward and Ubisoft, but that was only for spirits, which was probably all they could include at the time. Recall that Sakurai has stated that he mostly never deviates from the project plan when he's finished it.
So when time came to negotiate with Ubisoft or Wayforward, they couldn't get their characters in as fighters, but they could as spirits since they're just PNGs with stats attached to them, which makes them easier to include, project plan be damned. But a DLC Mii Fighter costume? Unless that company is really easy to negotiate with, ie. a Japanese company like Konami, there's no way they'd do that all for just a simple costume. It would be like them going to Microsoft for just a Banjo or Minecraft costume, and they sure as hell didn't do that with Banjo.
And with regards to Bethesda? There are plenty of companies literally throwing themselves at the opprotunity to have their characters represented in Smash, it's one of if not the biggest celebration of video games in history, anyone who's anyone is in this game. You're telling me that Nintendo came to Bethesda to use their characters, and Bethesda for some reason DIDN'T want to push for getting one of their flagship character in Smash as a fighter? They only wanted some Mii Fighter costumes? If you went to Wayforward, Yacht Club Games, Nicalis, IntiCreates, Playtonic, Ubisoft, Microsoft, Sony, or even Disney, and offered to put one of their characters into Smash as a fighter, they wouldn't hesitate to make that happen. and the same would go for Bethesda.