Free. No other explanation needed. Colleges and universities dropped all commercial operating systems when Unix came along so they could get the source code and then switched to Linux when they could get everything for free. That created a closed bubble where a lot of new ideas were forced to grow. but the operating system itself is still Paleolithic compared to what we should have by now, if OS companies like DEC, and others could have made money innovating their proprietary systems.
I liked it. An not just because there were very few other people that wanted accounts on it...
Exactly. No matter how big a business you are, there is always room to go cheap.
Incorrect. Most large universities and companies require vendor support. Initial cost is a secondary concern.