Consider that every industry has niche needs. I'll give some examples, but remember that my examples are only from a single industry out of hundreds. I work at a court of law. We have several niche needs that only can be satisfied with Windows software. Possibly the best example is our "Jury Instructions" program. Instructions given to a jury at the beginning of a trial will be unique depending on the nature of the trial. If a judge gives the wrong instructions, then a mistrial or appeal or even the wrong verdict can result. To complicate matters, the legislature changes jury instruction laws quarterly. The only programs that allow a judge to select variables about the nature of a trial, and then spit out jury instructions are for Windows.
If you consider that I'm just one court in one state in one country, then think about what a massive endeavor it is to keep software such as Jury Instructions up to date, and for what a small handful of customers. Nobody's going to go make and maintain a Mac or Linux version. It wouldn't be worth the effort. Now remember that I'm just one small tiny industry in an ocean of hundreds if not thousands of other organizations with unique needs. This is why we don't see anything but Windows in the workplace (that's a generalization... there are obviously exceptions).
The next evolution of court technology in my state is going to be a web based case management system. What do you suppose the dumbass developers did? They decided that people wouldn't want to install Java JRE, so the system is based in ActiveX. There goes any possibility of running it with anything other than Windows + IE.
[Rant]For the life of me, I can't figure out what the point of making a web based application is when you're locked into one browser/OS. Isn't the point of web based applications that they are platform independent? On the server side, I do run Linux/*BSD wherever possible. They run happy with 4 GB of storage. Windows Server 2008 typically needs 60 GB in my experience (due to WinSxS). You may think that storage is cheap, but not if you're running VMware Infrastructure on a SAN. I've literally had to quintuple my SAN storage in order to accommodate upgrading from Windows Server 2003.[/Rant]