You don't know that, and I seriously doubt you can prove it. Please don't offer conjecture as fact.
(Spoken as someone who does prefer Macs and Linux)
It sounds like the OP is interested in working with "latest and greatest" web technologies, though (no offense), and many of them are developed on Macs or Linux and therefore the best development environment for them is on a Mac or Linux machine.
I think it might just be a culture fit thing, as some people gravitate to .NET type stuff and some people gravitate to node.js or Rails, and just as it's a pain in the ass to get .NET stuff working on a Mac, it's often a pain in the ass to get things like Rails or node.js working on Windows.
Am I looking at a biased sample? Maybe, maybe not. I mostly operate in the web development world, but I know developers of all stripes. And none of them prefers Windows.
That being said, there are developers and companies who prefer to develop for MS platforms as a business decision. I.e. they want to target that market. But that doesn't mean they actually prefer developing on Windows. It just means Windows is imposed on them by their business choice. Analogously, I don't care much for Xcode or Objective-C, but I still write IOS apps because it's an important market.
All my personal opinion that applies to my particular circumstances only.
And the skilled developers aren't the young cats, they are the old dogs. Don't confuse what's popular with what's good.
Personally, I know just as many terrible old developers as I know young and just as many excellent young developers as old. What you're saying here is passing judgement on a group of people due to their age just the same as the OP appears to be passing judgement on a group of people due to their development platform preference.
Windows developers are out there, they're just probably not attending the same talks as you are and probably not reading many of the same blogs.
Best is what works for you in whatever role you happen to be in. I know people using windows coding in RoR, Python, Java, etc with a high degree of efficiency. I know others who are doing the same thing on Linux and OS X. It works for them.
I agree with the only caveat being if the dev shop is working on a platform-native stack (like .NET or iOS) - in which case you obviously have to use the 'family' platform for consistency and to be able to obtain all features (like being able to compile + emulate an iOS app - which can only be done on a Mac).
Do you have any data to back that up or is it just an uninformed opinion? In my experience the majority of skilled developers use windows but I wouldn't try to extrapolate from my experience (as a windows developer) to a univeral truth.
There are reasons for mandating windows (or osx or *nix) devices. Whether they are valid reasons depends on the circumstances. Personally I prefer to develop on the system that I am developing for but that is not always feasible (ios) or desirable (anything involving xcode).
>And all twenty-five coded atop, um, Windows machines. Facebook offered a choice of Windows or Mac, and according to Alves, no one wanted a Mac.
http://www.theregister.co.uk/2011/03/12/facebook_hacker_cup_...
For a company to say "at our company, we use Windows" is to say "We don't give a shit about developers". The issue isn't Windows, the issue is a workplace culture that places the preferences of management over the preferences of developers. Life is too short to work for people who don't respect you, especially when you've got one of the most valuable skills in the world.