So far none of the responses have gone on a business tangent, yet usually business guys are the ones who decided on / enforced the MS-only doctrine. I assure you some MBA veep who's never touched a keyboard did not make the decision based on the accurate technical details in some replies.

The first one is hiring. Supposedly everyone knows windows, great hiring pool. I donno if I believe that anymore, but in the old days there was certainly no shortage of MCSE to hire. It doesn't matter if the UI for the OS and office software change completely every couple years because "everyone knows windows" so training costs only exist for alternative OS or alternative office software. When that linux genius quits he's irreplaceable, isn't he? That's a great position to be in if you're that guy (me), but business people hate that. To some extent this is a young person vs old person problem because college campuses (campii?) are owned by apple, not MS. MS is just to run the dorm xbox, that's all. This is strategic planning on the part of decision makers.

The second is financial embrace and extend. Once you got one windows server or a couple desktops, inevitable licensing agreements make the marginal delta of "another windows box" extremely cheap (maybe zero) vs the measurable costs of mac etc. BSA / etc audits will make your life a living hell unless you participate in this scheme. This is bean counting on the part of the decision makers.

The third is interoperability. Unless you're the worlds most boring company you'll be doing something weird and under the first topic above, everything weird has by default windows clients or drivers or applications. There will be a windows client for an eprom programmer or I2C driver. There might be alternatives but it will work under windows. Then see the topics above. You will need windows boxes... you just will. This is pragmatism on the part of the decision makers.

The fourth is CYA. Like decades ago, "nobody ever got fired for buying IBM" today "nobody ever got fired for buying MS". You could get fired for buying a mac and it doesn't work, or installing linux on a server that crashes, but no quality of service, including zero, is low enough to get fired for buying microsoft. No matter how badly the system gets powned or how much stuff gets stolen, you can't get fired if you bought MS products. Just doesn't happen. If you can afford it, and you like being employed, the decision seems clear from a risk management perspective. Cowardice on the part of the decision makers.

The fifth is support. Business people think we get support by calling a script reader in India who tells use to wipe, reinstall, and reboot, and furthermore that is worth money and worthy of making procurement decisions. In the real world we know we use google for support and non-MS stuff googles better. But business people like a risk management of a nice security blanket of telephone support from a dude with a nice looking certificate. Its not actually useful of course, but business people don't understand it. Business people think computer support is like medical support and you call 911 and get fixed up, but 911 converted to computer support would be a pretty funny standup comic routine. Essentially this is plain old ignorance on part of the decision makers.