It seems like Pit has become as eponymous as say Donkey Kong or Yoshi in Smash nowadays, even if his first Smash was Brawl.
He gets in tons of advertising, he was announced in the original trailer with the original characters, Villager, Mega Man, and Bowser..
It feels like Pit has really become as important as the original characters, more so than even Zelda or Peach or Marth.
Pit is even on both Boxarts, while DK and Yoshi don't even have that. Jigglypuff also seems to be becoming less and less important as the entries go on. Especially since that whole, "Only revealing default characters before release" thing has been thrown out the window.