For Once, the public examines correctly!
The public has examined and analyzed the choices correctly.
Vista from a security and interface perspective is superior to XP.
But the public won't universally pay the price, low performance and scant driver support.
Linux and Mac's though arguably better OS's just aren't the mainstream. Users have Operating systems to run applications and Windows applications are, for the most part the applications people want to run.
MS has been hyping Direct X 10... there is nothing written to DX10 that anyone wants.
This all could change, but till it does, XP will be the OS of choice for the vast majority of users.