They're also aimed at different markets. Take the Brompton and the Airnimal, f'rinstance. You can fold a Brommie to a neat, non-oily compact cube in no time flat and stuff it on a luggage rack on a train. An Airnimal can go into a standard (ish) size case for plane travel, and is a fully raceable bike. Try racing on a Brompton, or folding an Airnimal inside 15 seconds and you'll be disappointed.
This is in no way to criticise either of them - they're both superb at what they do, but they're designed with very different aims in mind. Same for most folders.