Because it's so easy, there are a factor more Microsoft admins around than there are Linux admins. And they're fairly productive.
Plus, Microsoft software works really well together. Windows + Windows Server + SQL Server. Works great.
Linux is great if you have highly skilled technicians to support it. There's way more tweaking involved than with Windows; this makes it ideal for anyone running a centralised model where they're deploying thousands of identical machines (like the web). It's not so great for corporate environments where there's a high level of variance..
It's basically the Ford vs Ferrari argument: Microsoft enables companies to employ average quality labour to produce a "good enough" result. Ferrari employ top engineers to produce an extremely high quality result.
Ford scales, Ferrari doesn't.
Turns out after showing them they liked it and they are currently running the Linux servers. Windows machines are used throughout the company for the regular users, but Linux for their servers storing all the patient information files, user data, etc. After spending about 3 months as an intern I essentially taught the Microsoft certified staff enough to manage the servers well enough (it's really not all that hard) and they still have them running (roughly a year later).
Point is, you really don't need highly skilled technicians, none of them had used Linux before my internship, yet all of them still seem to run the servers fine. One REALLY nice thing is that their Linux servers go down far less, are quicker, and are actually more secure than their old Windows 2000 servers they were running.
Recently, I had to set up a development environment for ASP.Net development and was taken aback by the sheer amount of things I needed to install and knobs I had to turn to get it working. I've never had so much trouble on Linux nor OS X.
The same is true for getting a working build environment for our main product (CPython-based). I find Windows to be quite hostile to non-VS environments.
It's time to retire ye olde Linux-is-complex argument. It really does not hold up to scrutiny. I say this as someone that uses Windows 5/7 days and OS X 7/7 days, so I'm at least a little bit informed about the OSes.
It doesn't compare as well to Postgres - but the better tooling may well make it worth the $10k in licence fees. That's a month of engineer time in maintenance.
SQL Server's cheap for what you get when compared to the commercial competition. Informix is pretty much dead (and had horrible tooling). Oracle is even more expensive. And compared to Oracle, Microsoft look like angels. Ingres is a bit of a joke.
The big expensive MSSQL licences also include some pretty nifty BI tools, including decent implementations of data mining algorithms.
In server markets Linux + Unix have roughly 65% market penetration and it's growing.
Windows Server has 35% and it's shrinking.
I use Server 2008 R2 (and upwards) at work and its really not bad, it's miles ahead of the previous versions which were horrible to use. Is it not possible that people use this stuff because its useful?
And my point was perhaps the reason people are using Windows server is not because Linux is too hard and unfamiliar but because just maybe it's a better fit.
If everything in the MS ecosystem "just works" then I am either willfully overlooking its great features or like so much else, it's a function of what you know best.
There's little to gain by switching to another OS if you have enough money for licenses and you know its ecosystem well.
This might be more contentious, but I believe that people that are still using Windows Server (and Microsoft offerings other than AD) are doing so because it's easier to use/more familiar than Linux/open-source alternatives: Throw Joe-Linux on Acme-Cloud-Provider and use Weekly-Flavor-Config-Management? Sounds hard. Click some buttons and fudge your way through group policy? Now we're talking something I can do to pay off a mortgage.