weren't they added as dlc after? unless I'm forgetting something wasn't decidueye the only new character in the base game?
The point is, Decidueye isn't the "only" new Pokémon for DX.
But even without the DLC in account, that doesn't mean quite as much as you think it does.
It wasn't a matter of they went out of their way to promote Decidueye through Pokken DX.
Rather, it's clear they were going to promote
Sun and Moon with Pokken DX using the Starters, considering that a Litten/Popplio Support Set was added along with Decidueye.
So really, it was down to three options.
Decidueye with Litten/Popplio Support
Incineroar with Rowlet/Popplio Support
Primarina with Rowlet/Litten Support
And of those three options, they went with the first. Why?
Primarina, for starters, would have been too difficult to work with. So that option's out right off the bat.
And between the other two, they clearly went with Decidueye because it was the less obvious (and thus more enticing) option for Pokken's fighting system.
And yet I rarely see him discussed as a possible candidate either in here or even in places like YouTube.
Because it's not the
fully evolved Grass Starter and people can't fathom any non-fully evolved Pokémon being included in anything these days unless it's Pikachu (or Jigglypuff, in the case of Smash).