According to the US Census Bureau, over 80% of the US population lives in urban areas.
Again, folders sell in the USA - just look at that list of brands in post 1 - but there's precious little diversity in folding design.
Ah but how many of those people commute via bike? Not a large portion, apparently. Living in urban areas does not automatically translate to riding in them.
I suspect most people would choose to ride in the countryside over built-up areas if it was purely for leisure.