The build-up was definitely there. Sonia's side plot and a bit of Rose's builds this narrative that there's some extremely dangerous and powerful force that almost destroyed Galar before Zacian and Zamazenta stopped it. A force that could also solve Galar's energy crisis for seemingly forever.
I don't know. Personally, I don't think Eternatus has strayed too far from the likes of Zygarde and Necrozma at this point in its life. It's debatable whether it's been treated better or worse, but I don't think any of the three have stuck. All of the Gen 6 Legendaries/Mythicals have struggled to find a foothold in the fandom compared to others before them (Gen 3 and 5 more specifically). Zygarde was lucky enough to recieve a decent arc in the anime, but it wasn't so lucky in the games. Necrozma isn't very popular among fans (ranked 27 within its Gen in the Pokémon of the Year poll), which is probably why TPC has taken such a liking to Zeraora, their most popular Gen 7 Pokémon among Legendaries/Mythicals/Beasts. I feel like Necrozma has the advantage of being the face for USUM, something that Zygarde and Eternatus lack. Aside from that though, it hasn't been utilized very often for promotional purposes.
Maybe there's an unseen love for Eternatus than we're unaware of and we just haven't seen TPC take action yet. I have my doubts, though I'd love to be wrong.