I'm not sure this is the case. Source code availability for current and historic versions is what has made Linux so popular for studying OS design. Linux, the

I'm not sure this is the case. Source code availability for current and historic versions is what has made Linux so popular for studying OS design. Linux, the BSDs, the Mach kernel and many other embedded and academic OSes more interesting than Windows. Incidentally, even in the 80s and 90s you could get access to Unix OS source code. So in short people study what they are allowed to study.