It just works for the way most people I know use PC's. Usually I find they have at least one program they need that only works on Windows or maybe Mac. Might be Office (yeah, with Wine you can sort of get that, if you have an installable copy of a version WINE supports). Might be iTunes (although, not being an iOS user, I don't know if there's a version of iTunes compatible with Linux). They don't want to be told to use something else even if there's an alternative, they just want to use what they know. Most of them are accustomed from a young age to the Windows or Mac UI's (ironically also a bit of a problem for Win 8, although that does retain a somewhat traditional desktop). Might be one or more games. And when they come across a program they like on the web, they like to just be able to download it and install it, not try to find some Linux equivalent if there's no version for Linux. And when they install a new OS on their PC, if they ever do (and most never will, so that rules out a huge chunk whose only option is to buy a more expensive pre-installed Linux PC), they want all of their hardware to just work and not have to search around for open-source drivers and workarounds or go out and buy a new compatible piece of hardware.
This is what I mean when I say they "just work". Yeah, they've got tons of issues, they actually kind of suck frankly, but people are accustomed to them and have evolved a way of relating to their machines that doesn't always translate to Linux. And if I think someone doesn't need Windows or Mac because they're only using it to browse the web and maybe view some local files, I tell them to get a Chromebook or install Chromium, because at that point they don't need a full-blown desktop OS so they might as well just skip the middle man and go straight to the browser. Browsers "just work", again because people are used to them and have habituated to using them.
That is why I say Linux, or rather GNU Linux, doesn't "just work" for an average user, because those who live on the web and maybe view some local files would live easier with a browser-based OS like Chrome, whereas those who need and/or are unwilling to change their habits relating to the OS's they're used to will not find it valuable, and even someone who does love to customize and fiddle with his OS like me can't see the purpose in it because it has no programs I need I can't get another platform (often easier) and since I live primarily on the web the only arguable benefit is security, and I practice safe browsing and use malware protection and other forms of security for that purpose and haven't caught any real malware in years. So, apart from hobbyists or people who just subjectively "prefer" Linux, I don't see where it fits in the market. Yes, had history gone somewhat different that might not be the case, people might be habituated to Linux, developers might be focused primarily on Linux. But as an American driving in Britain will be unaccustomed to driving on the other side of the street and will likely be glad when he gets back to where the roads are "normal" to him), so will many Windows and Mac users be grateful to get back to their own familiar territory.