Not a source, but it's more common to have heat pumps in the southern States where winters aren't that bad. I'm born and raised in Arizona and they seemed to be common in houses after the 1973 energy crisis. My grandparents house in Tempe, AZ (was built in like 1950's) did no have one. A house I purchased in central North Carolina had a heat pump. I'm now around Washington DC area in Virginia, and we has gas. I think a simple explanation is that natural gas is almost a by product (certainly from landfills, which is how most USA disposes of trash) and is very cost efficient. Also the energy crisis changed the outlook of house building. I hope that helps explain it some, sorry I don't have any concrete facts I'm typing on a small keyboard.
