Yeah, the game being on a console most people actually want to own is a big part. Though also, come to think of it, what was special about Smash 4 leading up to its release, other than that it was just a new Smash game? None of its content or characters were particularly outstanding. Mega Man was the only huge character in the base game. The only major appeal of Smash 4 is that its gameplay is very good and it was HD, but otherwise there wasn't that much to get hype about. Smash Tour? Really?
Meanwhile, Ultimate is on Switch, has a bunch of new characters people really wanted, and has almost all the content from previous games and more.