Because despite the claims of Brompton fanatics, the Brompton design is not necessarily "the best". It entails many compromises in both rideability, serviceability, and weight to achieve a fold which isn't even the smallest.

I think most manufacturers know that the customers who find the Brompton appealing will generally buy Bromptons (often for reasons of brand loyalty and business/manufacturing practices/locale as much as design). So they make what they think will appeal to the remaining potential customers who demand other things such as better ride qualities, lighter weight, more compatibility with standard parts, more current manufacturing techniques, more economically sensible spare part policies, and even smaller folds.

I, for one, can tell you that I wouldn't buy a bike with a Brompton-type mechanism no matter who made it or what the cost because I don't like the feel of the rear suspension or its implications on the drivetrain. I think there are better designs out there which are manufactured with superior technologies and I don't put much value (as many Brompton customers do) on the fact that the Brompton is made in London and equipped with obsolete spare parts which are sold at a premium through Western distribution channels.