Well, the USA tandem, recumbent and bike trailer markets might be small, but even post-great recession they're still very diverse.

And folding bikes do sell in the USA, it's just the market is dominated by bikes that fold in one particular pattern. Most other folding designs can't seem to establish even a toehold through so much as a single, enthusiastic importer.

I dunno. Maybe for some reason the USA folding bike market is particularly price sensitive, and the Dahon-method is a compromise folding design that can be produced and sold relatively inexpensively.