I think that depends on the person to be honest.
My main reasons are:
* I have a rediculous amount of licenses for Windows and it's server counterparts ranging from 2.0 up to 8
* Ease of usage. Setting up a windows box is dead easy for CGMiner and ATI cards. Just install windows onto a VHD, set it up in BCDEdit and then boot to it. Stick in the ATI drivers and afterburner, cgminer, drop your conf file like usual, and if you have to, force a hardware overclock. Setting up a linux live distro with permanent storage on a usb stick when you don't have any optical drives, and getting drivers etc on to it involes more effort.
* If using windows to go, do the same, you can then duplicate the stick and put it in any pc you come across, regardless of hardware (unless you OC, then you have to OC each PC differently)
* if you are using the newer generation with any of hte newer Core intels (and I Think some of the AMD processors), you'll also get better power management. Although 5 kw/hr isn't a huge boost, but it helps.
* I play games on server if I can't be bothered rebooting. Just come on in, and let my games run through an NVidia card I have attached to the rig. The AMD/ATI cards mine, my NVidia card lets me play games through steam.
* HyperV, RDP support and software - I can use the same machine for work when I need to, so I don't have to stop any of my machines from mining when I work from home.
Additionally I'm an engineer, so apart from being able to design hardware for better cooling etc, I also program - and for the past 10 years I've primarly dealt with Windows and Server coutnerpart platforms, so when I want to write some UDP controls, or modify some hardware to remotely reset computers (instead of having to use ILO through a HP server) I can do that too, or jsut control CG miner, and give it a nice pretty gui to boot in comparitively less time that it would take me to write the same for linux. I also find it easier to automate and rewrite whatever I need to.
I also use any machine in an idle state to run CGminer or Cudaminer over SMB, no rebooting required - just let them go.
These reasons would be different for different people - but in general from what I've seen people like using windows due to ease, and practibility if they are multitasking (ie using it for mining AND something else they would normally use their PC for).
Windows Server editions compared to the non server editions can be similar or different however.
2008 R2 is SIMILAR to 7, but not everything is the same. However in general most stuff will work in both.
Windows 8 and Server 2012 are more closely related however as they share alot more common code - and an annoying metro start screen. So you can basically use either of them after you uninstall all the metro crap.
As for stability - I've actually found that the ATI drivers run better under Server R2SP1 than Win7 when overclocked.
Under R2SP1 each of my gigabyte 7870s get around 550 to 600mh/s using cgminer for btc. (I had a sapphire, but it was pretty much a dud when it wasn't underclocked so I got rid of it for about $60 profit). I can't overclock it to that point in Win7 or linux live (never botherd to boot into win8 on that computer as I don't have guild wars 2 installed on it), once they hit around 1150mhz they start to die and don't mine above 470mh/s stabiliy.
So, casting a general assumption, I'd say that server R2SP1 appears more stable than windows 7 for ati mining due to less load on the GPU (my win7 build is overburndend by a rediculous amount of software - it's been running since 2 weeks before public release on a sig edition win7 build). It appears to run better than linux out of the box due to availability of the newer ATI Beta drivers which seemed to fix a bunch of errors that my cards suffered on the release builds.
So that in general could be a few reasons why some people prefer windows.
Another reason is that people can't be bothered learning (or re-learning linux) after using windows for so long
