I think that the whole thing about the pumpkin spice phenomenon is that it's intrinsically tied to autumn in the US. Pumpkin pie, Halloween with pumpkins, apples, bonfires, etc. it's all part of the season, and pumpkin spice flavoring is just one of the ways that the season in general can be experienced.

Socially, it seems like white women, more than most other demographics in the States, are especially enthusiastic about celebrating and experiencing the various seasons (and is that really such a bad thing?), and since the autumn season has so many specific items and trends that go along with it that appeal to this demographic (picture the stereotypical white girl in the fall: PSL, fall style, at the farmers market or apple picking, family photos, etc.), it's just the most distilled example that gets tied to a type of person.

Other people get hyped for fall too...lots of rural American men are excited to go hunting in the fall...but camo and blaze orange with an old Stanley thermos of cheap coffee in an old truck at 4am isn't quite as marketable as the PSL.