feitsbob - CM Watson may have sold different designs of folders in the past, but all they advertise now is Brompton.
noglider - IIRC, NYCEWheels used to sell Birdy and Strida and Pacific Reach folding bikes, but they don't any more. I wonder why?
A Dahon guy said on that video from Eurobike that 90% of the world's current production of folding bikes uses the basic folding pattern from Dr. Hon's 1981 patent application. If that's really true, maybe folder diversity in the USA isn't all that different from the rest of the world. Perhaps I'm just having trouble accepting that's the design that ate the market?
those bikes are expensive, esp the pacific/birdy - all over $1000. the strida appears to be even more impractical and niche for a bike.
brompton is expensive, but it benefits from having a certain amount of cachet and a nifty fold.
dahon makes it because even though it is niche, many of the bikes are still affordable, comparatively. tern too.
it comes down to $$$$. imo, it's as simple as that.