Simple. I use what is best for the job and/or has the widest installed base and the software I want/need. And that is Windows on the client and Unix on the server for most of the stuff I have done the last 15 years(3). Linux is not good enough on either platform.
Servers: Long term (10+ years) stability is a must have for APIs, Drivers, Libraries etc. Availability of certified hardware from a big vendor is as well. Not having "Distribution wars" makes hiring Admins(1) easier and installing commercial software like RDBMS as well(2)
Clients: Support for all hardware our customers use. That includes notebooks and tablet pc since the early 2000s. Also central software delivery, company wide policies etc. In some cases (not all) our clients also use software that simply requires Windows.
With a 90+ percent market share on the desktop and a useable platform for tablets Windows is the easiest target with the best chance of sales. Add in that it is also the easiest target to develop rich / fat clients that work on Macs and Unix as well and our customers demand those client type for certain tasks. Oh and most use it as an internal platform (the rest uses Macs)
Running J2EE stuff is bette, faster and with less hassle on Solaris same for stuff like Oracle RAC. And most of our customers use that (or AIX) anyway on servers so Linux is a non starter
I don't hate Linux. It was evaluated (and is basically once a year) and found lacking for the needs of my employer so it is not used there. And for my privat use I see no benefit(server) or can not use it (Client - no penable support(4))
(1) I am not an admin by trade. I can do the job if needed (and did) but my specialities and certifications are in software, mainly Java
(2) With Linux the only "will work" platforms are those that come with commercial priced licences for support negating the last Linux benefit
(3) Before that is also included stuff like Step5, ORG/M, QNX, RTOS/UH. Also worked with Win/CE and Android.
(4) Or one not good enough compared to Win7 / Win8 if I count the Samsung Note as a Linux