The exam software my uni uses for instance only runs on Windows & MacOS.
I would say this segment of @Iceblade02's post would be the issue, in that people are locked into these systems even if they prefer to use open source software. For example, my university based in the UK requires I submit my assignments in an MS Word format that supports Microsoft's annotations for the tutor to do all marking up and correcting/commenting on the paper there. There are ways to do the same thing with PDFs, but at least on my modules so far, it hasn't been an option at all. That's just for papers and such.
When it comes to exams where you're supposed to be answering the questions and submitting them as you go, there are schools that insist on you installing monitoring software so they can make sure you aren't cheating, which only tends to be available for Windows and Mac. I don't know how common that sort of software is outside the US, but it's certainly a thing.