I suspect that there is a scale of severity and that not everyone who has it gets diagnosed -- so it's not as rare as it appears, but also with the internet giving exposure to different medical conditions, more people are getting diagnosed. (And more people are being vocal about having it.)

Personally I don't have any actual proof that I have it, especially since a lot of the testing involves hypermobility measurements that are kind of impossible for me, but it would explain a lot of things. And it amuses the hell out of me...