Why are previous generation GPUs such as the GeForce 4 series and GeForce 3 series not supported under Windows 7 or Vista drivers?

Why are previous generation GPUs such as the GeForce 4 series and GeForce 3 series not supported under Windows 7 or Vista drivers?

The new features found in Windows Vista require a GPU that supports DirectX 9.0 Pixel and Vertex Shader 2.0. All GPUs before the GeForce FX series do not support DirectX 9.0.