The oldest areas on the east coast were settled before gas lines were a thing and electricity existed. So they use oil unless it’s a big city that paid to have gas lines installed.

This is why cities that grew a lot after gas was industrialized primarily have natural gas.

Remote areas will use oil, propane, or wood because they can be delivered by truck and heat pumps are a pretty new technology that hasn’t worked well in cold areas until recently.

The south has minimal heat requirements so they can get by with electric which is cheap to install but not efficient enough to provide primary heat in cold areas.

Also, southern homes generally have air conditioning so adding a reversing valve or set of heater coils is pretty easy.

One reason is bad laws. I would love to replace my natural gas boiler and steam radiators with an electric heat pump system. It would be more efficient, cheaper to run, be able use solar power, and keep my house more comfortable.

But state law requires that buildings meet a specific efficiency requirement, which my house does not meet because it is 170 years old. I understand why they have that requirement for new construction, but it is stupid to not have an exception for old buildings.

Climate. You're not going to need gas heating in Florida when it's cold for only a short time of the year.

Probably also a bit of a history component.

When my parents built their house in rural New York, they could have chosen gas or propane. However that was the height of the first wave of nuclear power, when it was being promised “electricity will soon be too cheap to meter”. Of course now that looks like an insane choice as the most expensive.

Any areas built out during the 1950s-1980s may have followed this logic

Age of the city and sufficient infrastructure. Once the building is built to use [oil or gas or whatever] it’s never refit to use something else. Something something quarterly profits.

Natural gas here, because the city has an extensive network. Previously, living with my parents a city away, the condo had oil heat.

A year and a half ago I had to replace my cast-iron boiler and (leaking) hot water heater, and so I took advantage of financing to replace them with a gas-powered tankless combo.

Market supply and demand I assume.

Age of home probably plays a factor as well.