The same reason most organizations use it -- inertia and because it's been the standard for so long, it's the best at what it does.

The startup I used to work at was exclusively on OSX + GoogleDocs, when we were small, but as we grew (and especially when the Finance team grew) more and more employees found a need for the MS Office Suite as well as apps that only run on Windows, so they started rolling out Windows VM's and then full Windows machines.