Because people like to see their favorite Nintendo represented in a Nintendo crossover game. What's the point of a new Smash if you don't get any of the characters you want? I'm glad that DuckHunt and Bowser Jr look fun, or I'd probably hate the game.
It's not "any of the characters you want" though. If there's a guy out there heartbroken over the lack of every single one of his dream characters, then my heart goes out to him.
But there's not. There's people that are clinging to Ridley and need to let it go and enjoy the video game's many other characters.