More seriously though, it is difficult for many people to application software. Switching from a commercial to an open source operating system takes that difficulty to the next level since it usually means changing almost all of the software that you use. (Unless, of course, you use open source applications under a commercial operating system.) I suspect a lot of people like to advocate Linux to those using older systems because those people have less to lose.

It is easy for existing users to promote installing Linux on the best computer because we are already familiar with its strengths, we know how to deal with its quirks, and we know that it fits our needs.