According to an online survey by Valve Software, only one in fifty players who access download service Steam has a DirectX 10-compatible graphics card and Windows Vista installed. In an interview with heise online, Gabe Newell, president of Valve Software, said that Microsoft made a terrible mistake releasing DirectX 10 for Vista only and excluding Windows XP. He said this decision affected the whole industry as so far only a very small percentage of players can use DirectX 10.
When developing cross-platform games which are also released for Xbox 360 and Playstation 3, developers look for the smallest common denominator. And since neither Microsoft's nor Sony's new consoles support Shader Model 4.0 for DirectX 10, only a few games use it.