The dirty truth: Marketing.

Back in the 90s there was a major push to market Linux by a wide number of vendors from Red Hat to IBM if I recall correctly. In the dot.com 1.0 era you couldn't leave a tradeshow without being given a t-shirt or stuffed toy of the tux mascot. You could also go into a bookstore, but a book on a book on Linux and get a free CD with an installer. You could also go into a computer store of that era and buy a boxed copy of Linux. This made Linux really popular with the first generation of web geeks.