Installing Linux, although in many ways superior (I think iptables is such a better firewall, no need for antivirus because of a good privileges model, Apparmor can be really useful, packages are amazing and almost every Linux distro does them pretty well) is too much of a hassle for the 90% of people that want a computer like they want a hammer or TV. It is a tool, you hit the button, something you want happens. Having to understand the entire thing is slightly more complex than that requires way too much mental exertion.
It is, in the end, why "Linux on the desktop" never happened. It was never the default. It was never on the Best Buy shelf when grannies 15 year old laptop broke and she needed a new facebook machine.