Being a Windows user having tried to get into linux, there is a lot more than user inertia. A few reasons:
The desktop is almost dead, what we call desktop today is really laptops, with custom hardware for which the drivers are rarely all available for linux. Try telling my mum: "you need to compile your own drivers".
The UI is extremely limited and you end up having to type command lines almost immediatly, to install a software or change a configuration. First if command lines had even a remote chance to go mainstream, DOS would still be around and Apple would have gone bust. Then, there are many distributions of linux that take a different syntax so it's not even trivial to find help on the web. And again, try shouting "RTFM!" to my mum.
Linux developers seem to be in love with cryptic acronyms which means nothing is intuitive or simple.
For Linux to take over it needs not only to match but to surpass its competition (MacOS, Windows) in term of simplicity and ease of use. And right now it is far far far behind.
I am sure we can make the list longer. But blaming user inertia for linux not going mainstream is exactly what Microsoft was objecting when they were told that the new Windows 8 UI was a piece of shit.