Here's a comment from another site I found pretty damn insightful that sums this situation up beautifully:
People need to stop and realize the historically poor economics and profitability of Linux to understand why the experience has been hindered in recent years. Yes, drivers have been historically bad for Linux where graphics adapters and gaming are concerned.
AMD/Nvidias target market doesnt overwhelmingly demand linux support.
and the gamers dont demand it because there arent many games for the platform.
and there arent many games for the platform because the driver support sucks and they likely wouldnt sell many copies since the platform lacks gamers.
Its a vicious cycle that requires at least one of these elements to change before the ecosystem begins to mature. Nobody is willing to take the first step because its almost certainly unprofitable for themselves to make that initial investment. Nvidia doesnt want to waste developer hours on writing a fully optimized linux driver if its just going to be used by less than 1% of their target market. The industry is too competitive to allow breathing room for philanthropic development.
Valve is breaking the cycle by getting the platform attractive to every single party involved. Theyre attracting the hardware OEMs by making the platform optimized, and investing lots of their own cash into the OS also making the experience familiar and easy to use (something Linux doesnt really have going for it currently).
They get the gamers into the fold with backwards compatibility (through the Windows steam client streaming feature) and content services. Theyre presumably releasing cheaper thin clients primarily for streaming content from your Windows system.
Finally, they can attract the game developers by the fruits of their labor with the hardware OEMs, demonstrating that the platform has begun to mature from a development platform standpoint and that there will be an established install-base of gamers who are steadily adopting the platform.