Why use Linux? Because of security! Because of control! Because of privacy, community, and a general sense of purpose! Because it’s fast! Because it’s virus free! Because i’m dang-well used to it now! Because, heck, I can shape it to look like pretty much anything I want it to using themes and widgets and CSS and extensions and blingy little desktop trinkets!
Security - unless you screw up one of a million subtle things. Control? Not only does it allow you control, it _requires_ that you understand how to control every damned thing (90% of the time it just works, then the other time everything is broken). Fast? Unless you configure something wrong. Virus free? Granted. Can make it look any way you want to? Well, yes and no - you can make it look many different ways, but you end up swearing at whoever forbade the particular combination you actually wanted, and every few years and upgrade screws everything up because new-GNOME has no relationship to old-GNOME.
because it’s better
Unfortunately, now that my desktop is set, I get cranky about my infrastructure services breaking every six months when I do an upgrade. Unfortunately, there isn't an alternative that I think will beat Linux for what I want to do. But at that point we're well into nerdville, so if you want to get into a heated discussion about Linux versus FreeBSD, great, but that isn't going to drive installs for normal users.