Regarding the lack of regular media exposure from the Mickey Mouse company(and others) for Giannis, I would ask, how has Milwaukee been as a draw on nationally televised games? Is it much lower than super teams in major markets? Is it a lot lower? That could be a reason for the lack of hype. His lack of exposure via being an advertising spokesperson could also mean the casual fan to non-fan wouldn't know who the heck they were talking about. That doesn't help the situation, that's for sure.
Regarding the lack of adoration or exposure on social media, Giannis has a very small presence on social media(as compared to other sports superstars). I don't believe he has major personal business ventures that would almost demand him to use social media for exposure, though maybe he has some philanthropic ventures he should be on social media for. Also, does Giannis have as many NBA personal connections, as others. Lots of NBA guys grew up knowing each other since AAU basketball, so other stars just may have a wider base of NBA friends that would laud him on social media.
I am not so sure there is any nationalism/xenophobia involved here. I just think it's a numbers/familiarity/bottom line situation making it look that way. It's easy to blame divisive emotions and intentions, but I think there are definitely other factors at work here.