Microsoft adds costs
Windows is Legacy Windows-NT based largely on VMS. And Windows 8 is based on Windows NT.
And VMS is basically a dead branch except Windows and 'OpenVMS'. VMS was developed for the Vax PDP in the 1970's, by David Cutler who was smart, but had a disdain (personal gripe) and was anti-Unix. Linux was based on MINIX and developed in the 1990's, now many companies use Linux for devices, desktops, servers, High Performance (from a raspberryPi, android, desktop, Tivo, SGI, Sun, IBM and Cray). Also its close cousin OS BSD used on the Apple Macs (which versions of BSD run anywhere).
Everything else is Unix/Unix-like is in what _everything_ else runs. (Linux, Android, BSD, Macs, most servers and really all HPC).
Microsoft SQL would be extra due to:
- Extra hardware overhead running Windows
- Extra cost running Microsoft Windows (license per instance)
- Extra cost running MS SQL (license per instance)
- Extra cost of security issues/flaws in MS Windows and MS SQL server
So Unix/Linux is anything bug Legacy, and the extra cost makes sense. Perhaps higher than it should be, but obvious why it would be more than free, open source, intensely tested server software. There are bugs in Linux also, but releases are often before exploits, and reboots are often not required (just a restart of a service)