Theories are cheap here of course. I think a main technical factor is that the devs and middleware authors interested in the platform have been constantly kept

Theories are cheap here of course. I think a main technical factor is that the devs and middleware authors interested in the platform have been constantly kept waiting for the next platform features just around the corner, and making big games is a long, 5-10 year process. But then there's the business side. There's a longish history of web games and there are game genres that don't need 3D acceleration, but there still aren't many (any?) profitable web games.