Misery loves company. You won't see anybody telling people to switch to Windows, Mac or iPhone because those people are too busy enjoying their OS.

Not really, Linux evangelists are a thing because Linux is primarily held back by lack of adoption. A lot of common issues can be attributed to either:

1. The hardware manufacturer has never tested Linux support for drivers. 2. Some application that you need doesn't target Linux due to lack of users

This isn't everything, sure. But I think it's a majority of the headaches. Thus, Linux-users really want other people to also use Linux, so that companies actually give a shit about supporting it.

There's also the whole ideology involved. A lot of companies are increasingly pushing that you are not allowed to control the computer/phone/device you buy and Linux is at the forefront of combating this.