Zeraora was discovered via datamining WAY back in 2017.
I mean a mythical Pokemon who plays a major role in a movie seems like a pretty good rival for that. Also he's a Pokemon from the most recent Pokemon game. Seems like a good promotion for Ultra Sun & Ultra Moon (which this Pokemon is exclusive to.)
Oh, yeah, I know it was discovered back in 2017, it is just that
officially it has only just been revealed, so the general Pokémon community, who would not necessarily follow any Pokétubers or check Serebii would not know about it until now.
It does have a significant movie role, but so have many other legendary/mythical Pokémon. I've just checked and the movie also does not release in Japan until July and there is no official English release date, so the movie is still a fair ways off, hence so would the distribution.
I think in terms of it being a marketing push for USUM it is pretty far past that since we will be getting into Gen 8 hype from E3 onwards. So we will more likely see representation over hype material, which Zeraora does not necessarily fulfil since it was not in the original Sun and Moon games at all.
That being said it is still possible that Zeraora will be the Gen 7 rep, and I would not be opposed, I just do not think it is that likely.