Because 'one-size-fits-all' just doesn't work...
Much as I dislike M$, there are actually situations where a company would be better off (at least in the short-term) running an NT/2000 server.
It's much cheaper for a small company that doesn't have competent IT staff to run a small NT network than it is to run a unix-based network. Particularly when they have mostly M$ client PC's.
Unfortunately, most companies don't recognise the fact that once their network has grown beyond a certain size, M$ server products really are no longer efficient - both in terms of dollars and performance. With a larger network it's cheaper to employ experienced staff and implement better solutions (with correspondingly lower administrative overhead) than to just throw more hardware and licence fees at the M$ network.
Also, in many cases Windows is not the best solution for the desktop either.
The Mac has proven itself the leader for desktop publishing and graphic design.
SGI provide one of the best platforms for very high-end graphics / simulation / 3D rendering.
For open-standard application development, Linux is emerging as the clear leader.
If you want security, flexibility, and stability you'll use a BSD flavour.
Mostly the choice is yours... whatever you find suits your needs best. The main point here is that we do have a choice.
If M$ had their way, we would have no choice...
