Well it's not just like any other beat-um-up game its a totally unique version where real time strategy is as important as fighting.
If you think button mashing skills will help you you will be thinking it as you come in 4th place everytime. The most damage inflicted does not mean you win in this fighting game.
The truth is you can't come up with any other game quite like SSB. So don't get crazy and just blow it off as something you could get better somewhere else.
Does it appeal to everyone? Of course not, there isn't a thing in the world that appeals to everyone , except maybe water lol.
But for a fighting game it has to have the most diverse demographic "a.k.a. universal appeal" compared to any other fighting game on the market. Fighting games in general are huge niche style games with die hard fans, This is one of the few that gets all kinds of different people playing it because of its setup.