It's true that there have been a lot of changes and advances in hardware and in OpenGL; keeping up with these takes effort and new ideas.
But if you go back a decade to when Linux graphics performance and hardware support (Utah-GLX and DRI) were closer to parity with Windows, things weren't simple then either. There were dozens of hardware vendors rather than just three (and a half, if you want to count VIA), each chip was more radically different from its competitors and even from other chips by the same company, etc.
While there's been progress in an absolute sense, relative to Mac and Windows, Linux has lagged significantly in graphics over the past decade. Graphics support is a treadmill; Linux has has often been perilously close to falling off the back of the treadmill.
I don't mean to say the efforts of those working on the Linux X/Mesa stack alphabet soup have all been pointless; nor do I claim that all of the blame rests with them. The ARB deserves a lot of the blame for letting OpenGL stagnate so long. It's a real shame that other graphics vendors and developers from other Unices haven't taken a more active role in helping design and implement the graphics stack, and while I think more could have been done to solicit their input and design things with other hardware and kernels in mind, they're responsible for their own non-participation.