Bias towards Windows is an interesting one. As someone who programmed on Windows for 9 years, I got away with writing code in IDEs and since most apps had GUI and command line was less than worthless, I was never exposed to simple tools that could be chained together under a minute that one had to spend hours either writing, or looking for a software that did the same in Windows.
It wasn't until I started college, where I had to use Linux to get my assignments done, etc. that my mind got stretched open. In three months I couldn't believe I survived in Windows for that long.
As an employer, the fact that someone has only ever developed in Windows and also uses Windows at home, tells me that at best they can appreciate unix and unix philosophy, but that it's unlikely they could build tools that do the right thing and play well in a unix environment, no matter how many years of experience they've had building tools.
But not only that, unix goes deeper and teaches you about how to do things inside your program, even if that part never talks to other programs. Things like do the dumb thing first; or when you have nothing to say, say nothing (e.g. `ls` prints nothing if no files found), etc. It's about mindset and values and those are almost always dictated by the environment. Also values cannot be looked up on stackoverflow and copy/pasted in 5 minutes. They take a long time to sink in and I can understand why an employer would want to skip that cost when hiring.