AgentKallus
It's not a game Kate.
I suspect it's because Generation V is the most recent generation to be added to Pokémon GO, and there are some cross events planned. Galarian Weezing is already available in it (it's Generation I, but I mean that they're referencing Sword & Shield already), and I'd bet they're going to make Galarian Darumaka and Darmanitan available soon, as well as Gigantamax Garbodor and some other stuff.
Even as an active player of Go, I highly doubt that's the reason. While there's more overlap between Go and the main series than any other spin-off before it, there's still just no way that Game Freak went out of their way to design more stuff for Unova than they would otherwise simple for the sake of Go. That's just not the way things work. The main series comes first, then the spin-offs. It's completely backwards to think things flow the opposite direction. I'm sure we'll get stuff like Galarian Darmanitan in Go sooner rather than later, but I doubt Go was the reason for it existing in the first place.
I also play a lot of Go, and if they were doing the regional forms with Go in mind I doubt we'd have gotten one (+ an evolution) for Farfetch'd and Mr.Mime without Tauros and Kangaskhan getting similar treatment as the 4 are treated as counterparts of sorts in Go. Also I'd argue that gen 5 being the most recent generation added to Go makes it more unlikely that they were doing it with Go in mind as new versions of already new things are as exciting as new versions of new things.