Indeed, why downgrade to Linux if you can run FreeBSD :P
A bit more seriously though: cost reduction, continuity and customer value.
Microsoft wants you to upgrade to a new environment every once in a while. A long time, that is absolutely true; but when the time comes then Microsoft no longer cares about their customers any longer, then it's all about revenue. Look at where we are now: XP marketshare is even expanding near it's EOL yet Microsoft refuses to acknowledge that for many people the new dinkey toy servers just don't cut it (I'm now referring to the touch crapola which is Metro). TechNet? Thousands of Windows / Microsoft administrators (yours truly included) cried out in pain and were easily ignored.
As to Metro: sure; on Linux (or my personal preference of FreeBSD) stuff also changes. But in the end you are and remain in control.
Which brings me to continuity. Linux and FreeBSD are at the core still the same Unix-like commandline based operating systems which they were several years ago. Stuff got added, of course, stuff got removed and stuff got changed. But in the end it's still RPM / yum, DPKG / apt-get and well... yeah.. Those FreeBSD hippies had the audacity to actually change their package manager to something completely different. FreeBSD 10 doesn't use the same tools as previous versions, I guess Microsoft isn't the only one which drastically changes stuff.
I tell you: instead of typing "pkg_remove -x stuff" I now have to type this instead: "pkg remove -x stuff". Effectively replacing the requirement to type _ with a space, how horrid is that?
Which brings me to customer value. The powers behind these operating systems actually value and respect their userbase (generally speaking; of course you can always come across some weird guys, happens everywhere. But my point here: they don't force you to do stuff you may not want or like. No one is going to force you to install X (the GUI) on your servers for the only reason of being able to run their own software. Not going to happen.
If a Linux distribution does this and their userbase doesn't agree then they can (and usually will) run into problems. Because in general it's fairly easy to switch distributions (this holds especially true when looking at "descendents: Debian and Ubuntu for example). As to FreeBSD? Well, hardly anything ever changes there. It's still the main core operating system on which everything else gets installed "on top".
From personal experience: Microsoft would rather see that we (small company) upgrade our hardware, buy ourselves extra licenses (even though we only need the environment for internal testing purposes) and if we can't or don't want to then their other alternative is the "Cloud". Yeah right...
FreeBSD on the other hand (personal choice as replacement for our Windows 2003R2 servers) easily runs on our current hardware, can perform the same tasks as the Windows 2k3 server yet also a whole lot more too. Think about extensive IPv6 support for example.
Sure; Mono isn't fully up to speed with the latest versions of .NET yet. So if you have specific requirements then this obviously won't suffice. But for everyone else..