Well it's not that ATI for example doesn't support OpenGL at all or something like that. It's just about some extensions for specific features you have to write for your drivers and that's obviously a pretty complicated procedure especially if you want it to be performant too...

Apart from that the desktop market has only a very small percentage of people actually really needing that and that's why this has a rather low priority (although especially for ATI I've seen quite some additional attention on this lately)...

Things might be a bit different for the professional cards like the Quadro and FireGL series but here you also can see ATIs higher interest as in June there was a new driver giving you roughly a 33% increase in OpenGL performance so on this sector there's more time spent for this (as CAD and so on are quite important apps)...

Enjoy your meal
Toast