That's asking the wrong kind of question. Linux is just the operating system, more precisely the kernel. A lot of people confuse this, I remember reading "Windows 8 is so great, it has a cool new Task Manager and new features in Explorer" in another thread. The task manager is just a rather unimportant utility program, it is not the operating system. Explorer, likewise, is just a file manager, which is in principle swappable. The same goes for the task bar / start button or the new tile interface. These are as much "the operating system" as the desktop background image.
I'm still wondering why Linux still doesn't give a crap about casual computer users.
GNU/Linux on the other hand (note the "GNU/" part) is something that has "free Unix" (hobby) Enthusiasts as primary target group. Obviously, although not stricly opposite, this is not the same group as "casual home users".
You should be asking why there is no major distributor who cares about casual home users. Though I remember SuSE being like this, 10-15 years ago. It used to be "put in CD, click install button, and it just works, and works well". Of course that was at a time when Windows sucked ass, Apple failed big time on PowerPC, and no such thing as a GPU existed in a PC. SuSE at that time was a hundred times more stable and user friendly, and more performant than anything else (except for some unusable geeky Linux distros). No manual unpacking tar files from floppies, no editing /etc/fstab and /etc/password in vi. Insert CD, and there you go.
In the mean time, Windows has become perfectly stable and (at least up to version 7) usable, and in constrast to any Linux distro, every single piece of cheap (or expensive) hardware that you plug in comes with working IHV drivers. There is no thing that you can buy in a shop and plug into your PC that doesn't work with Windows.
Ubuntu used to be somewhat like "the new SuSE" until a year or so ago, and it even had accelerated graphics with no hassle (big, big plus). This changed drastically when everyone jumped on the "must make desktop like tablet" train, starting with Gnome 3, later copied by KDE and Unity, and finally stolen by Windows. Now all operating systems / distributions suck the same, because they have the same stupid user interfaces. Which would be fine if every computer was a tablet (then those interfaces would make sense!), but trying to make a desktop computer a tablet is just stupid.
I remember trying to install a recent SuSE version on a virtual machine not long ago. The wizard required me to click on a button somewhere on the bottom, and moving the mouse onto the button made the window manager zoom out the Window and show the action button interface or what it's called -- how fucking stupid do you have to be as a developer to ship a thing like that? I mean, they really don't want you to install their stuff, do they?