Operating Systems tend to live and die by the dedication of their users.
People, like you and me, grow accustomed to using computers in a particular way. How our file system is laid out, what the desktop looks like, what applications we use to get work done - over time we become used to every nuance of our systems.
When that system changes dramatically, we start thinking “hey, this annoys me... I wonder if the grass is greener over in that pasture over there.”
Windows 8 is a great example of this. As Microsoft changed the core user experience, folks (at least some folks) began to look elsewhere. Or when Apple first released MacOS X 10.0 and changed just about everything about MacOS (new apps, new look and feel, new file system layout, broken backwards compatibility). At that moment, many Mac users considered jumping ship.
This same phenomenon applies to Linux desktops as well, which is perhaps a bit counter to common thinking – many use Linux because of how astoundingly customizable it is compared to most other Operating Systems. But it applies, just the same.
For example: I used GNOME 2.x as my primary desktop environment for several years. I loved it. It was fast, flexible and, perhaps most importantly of all, I knew it inside and out. I could make GNOME look like just about anything. I could make that puppy dance.
Did I, as a good Linux nerd, try out the other desktop environments? You bet. I tried them on a regular basis, in fact. But I always came back home to GNOME 2. That was my desktop. That's what I knew, what I loved.
And judging by the extremely high popularity of GNOME 2, I wasn't alone.
Nowadays, things are a bit different. GNOME 2 is dead. GNOME 3 changed everything about the popular desktop. KDE 3 (the other king of Linux desktop environments over the last decade) is dead as well – with KDE 4, again, changing darn near everything you can think of. Add to this mix things like Ubuntu's Unity and you've got, to put it bluntly, a bit of a problem. We now have a scenario where Linux desktop users are, by and large, in a state of "desktop environment-less-ness."
We have no...desktop home. Many of us are looking around, trying to decide what to use, what to switch to.
For the first time in a long, long time, Windows and Mac users can look at us Linux users and say "Oh, you're not satisfied with your desktop environment? Maybe you should think about trying what we use..."
Of course, it's not exactly the same thing. The desktop environment is merely one component of what makes up a Linux desktop, and changing to another environment doesn't mean you necessarily need to change your file system structure, package manager, default apps, etc. But it's a slightly uncomfortable state to be in, just the same.
It also tells us something important: GNOME 2 (and, before that, KDE 3) scratched an itch that the current desktop environments are, perhaps, not scratching. Is that merely a symptom of the relative youth of these new environments? Will it gradually rectify itself as they mature further?