A couple of PNGs probably isn't gonna stop the CEOs of this juggernaut franchise from squeezing a bit more relevance out of their latest games. That spirit event had... what? 7 Pokemon? Out of the 90 new ones Galar has to offer? That's more than enough for a spirit board.
You know it's NOT about the spirits right? Because everyone knows they could follow up on that with more Galar/Generation 8 Pokemon.
It's more the fact Pokemon gotten several promotional events compared to the fighters that did get one afterwards and the recent releases and series that got on without a fighter. It's more of a planning case for whenever they probably don't have something planned later.
Pokemon was given 2 Spirit events for SWSH and LGPE, 2 tourney events to celebrate SWSH's release and Pokemon's 25th Anniversary, and SSBU x Pokemon was seen on top of getting those Team Rocket costumes.
Two of Capcom juggernaut series got passed over for events, both Resident Evil and Monster Hunter respectively with Sakurai admitting he wanted to promote Rise as soon it was releasing.
Ring Fit Adventure, Astral Chain, and Paper Mario: Origami King, all successfully sold switch titles, were slapped with events.
Animal Crossing New Horizons, a title that's been doing gangbusters on the Switch as of late, ended up getting it's own spirit event, but we already know that Isabelle was already tied to New Horizons promotion years ago, so another AC rep wasn't that likely for DLC anyways.
I am still expecting for a Generation 8 Pokemon regardless, but as someone who doesn't like to treat everything as a lock, it's definitely something to keep note of.
As for Let's Go's case, they would have just put in Melmetal or some popular Kanto Pokemon and called it a day in Fighter Pass 1/2, but they never bothered lol.