Burning natural gas is among the cheapest forms of heating we have, and it works 24/7/365 and at all latitudes. Solar heating using a slab as a heat reserve requires re-engineering houses completely and may be impossible in a lot of places that don't get a lot of sun during the coldest months.

Perfect statement of someone who more than likely lives within a city with public utilities. However, there are actually a large amount of people in rural U.S. that have to use either Oil or Electric heating (in which both are expensive). In These rural area's, Oil heating is the norm and in order to convert to an electric heat there are extensive cost's involved (which most people in these area's cannot afford). Living in a city (especially west-coast) has certain amenities that the rest of the U.S. does not. Complete solar power in rural Wisconsin or the Dakota's for example isn't a real viable option. There is much less sunless then the west coast and extreme season weather changes (humidity, rain, snow, hot/cold).

Solar panels heat well in sunny weather. Which is the opposite of when you need heat :)

Also need to design and build with materials that maintain as constant a temperature as possible. Double glazing, insulated walls, floors and roofs, orientation to the sun etc.

I'm fairly certain the rest of the world would be happy to exchange cash for oil or oil for cash.