Not every Smash Bros. player bought all DLC.
Most did.
The idea that everybody will eat it up is just plain not true. It should sell fine, but you can't expect sales from 100% of the world. Just enough to justify the inclusion. A sales expectation is a calculated risk. Nobody truly expects every person in the world to buy something as DLC. That's just silly. They still try to appeal to the majority to guarantee sales, though. That's actually reasonable.
The only people who'll buy all the character and stages are Tourney Owners. It's far more subjective for other players. Competitive players are likely to get it all(though some won't, which is silly, cause that means you don't have all the tools in the game to properly practice. But it's still their choice to do it that way. For better or worse when it comes to playing competitively, it's their money to use, not anybody else's).