Linux has hardly been used as a gaming platform. As a result, Linux drivers had stagnated for too long. NVidia and ATI were only interested in high-end CAD workstation markets, certainly not in Linux gaming.
With open source drivers it's even worse. For example, DRM2 and KMS were introduced after Windows Vista came out with its new driver model and DirectX10. Mesa has achieved OpenGL3 compliance only earlier this year.