Some quick Googling fails to find a good answer, but I'd venture to say that a significant majority of the US population lives in places with mild enough winter

Some quick Googling fails to find a good answer, but I'd venture to say that a significant majority of the US population lives in places with mild enough winter weather for heat pumps to be practical even without good cold weather heat pump technology. The northern interior US, where the coldest winters tend to be, covers a lot of area but is pretty sparsely populated compared to the coasts and the south, probably in part because most people don't like frigid winters. Arguably human temperature preference is actually ideally suited to favorable heat pump conditions.