It's very nice to give end users the option - when I joined, I asked for Linux and was given a brand-new Dell XPS with Ubuntu pre-installed, root access, and told, customise it to suit you. I keep that spirit with my users.
Throwing Windows into such a setup is a nightmare though - two users actually installed their company laptops with their own personal Windows 10 licenses without my knowledge or approval - I still hold a grudge because I had to actually read the MS EULA to make sure they weren't about to cause trouble! It means I can't manage them (I have no real tools to do so on Ubuntu), so they're on their own for that. Fortunately management has my back, declaring the company to be a *nix shop.
Our actual product is all containerised so it should run anywhere.
Ansible works pretty well for managing Windows machines but it requires a little bit more up-front setup on the Windows side and as always YMMV depending on what you're trying to do.
I suspect that it was on the local tooling side that things fell apart for this Windows developer, if he was grabbing a project mostly built on Mac or Linux.
A lot of popular dev tooling built around web technologies (node, Ruby, PHP, etc) isn't as mature on Windows as it is on MacOS or Linux.
My source on this is me. I have a Mac and a Windows box at home, and I've had projects fail to set up and build on the Windows machine for these reasons.
Maybe the developer in question would have been better using a manually created VM or WSL?
Not to mention, the windows environment is completely different to linux and OSX. Until x64, they had a different calling convention. On OSX and linux, also until recently, I could use the same compiler for both platforms and still be supported by the platform vendor.
All of these non-trivial differences make it a lot more resource-intensive to support a codebase on windows that already runs on Linux and OSX. Asides from my work VMs, I don't even have a windows device that I could use for development. So to me, it's no surprise that most open source projects don't build on Windows.
Also, mingw and cygwin also exist since late 90's.
OS X tools are kind of included on Apple's hardware price.
Apple hardware costs in many countries is similar to average PC + VS License costs.
As for mingq and cygwin, most regular people don't have it installed and configured. And cygwin and mingw are not _platforms_ which receive first party support, as unfortunate as that might be.
It was pure pain. Especially when the boss kept asking me what was taking so long (it's just a recompile right?).
500 € PC + Visual Studio vs 1000 € Mac.
All projects are ultimately created to scratch somebodies itch. If it doesn't work on windows out of the box that isn't their use case. What you are wondering is in effect is why people don't pay money to purchase a windows license which will ultimately fund a bad and evil company in order to enable the projects software to run on an inferior OS that the dev doesn't run or care about. If its not end user software for desktop users it doesn't even have the positive effect of enabling a substantially bigger group of potential users to benefit from the software. For anything server related they are either already running linux or can as easily run a linux vm as a windows one.
Further the users who would benefit will by and large buy a license if the software is non free but probably wont contribute anything but complaints phrased similarly to the support requests they would make for paid products that had failed to perform adequately.