Because they don't listen to reason and their solution for everything isn't to fix the problems with other OS's, it's to switch to Linux. And they're... Smarmy about it.
You don't know the circumstances of other people. Maybe there is something they need to do and it doesn't work on Linux or they don't want to spend time to work it out. Is Linux better for a lot of things? Absolutely. Is it better for all things?. No.