When I look at all of the major variants mentioned - Gnome, KDE, Windows, Apple - I honestly don't see any great difference.
All of them offer:
- A desktop
- some kind of task bar (top, bottom, left, right - doesn't really matter)
- some form of menus for getting to stuff
- some kind of file manager application
There may be some things that are very different from one to the other (Lord knows that when I switched to a Mac I found some of their choices thoroughly obscure) but in the big picture most desktop systems are similar enough that Joe User can go to one or the other and figure out how to check his Yahoo mail account without problems.
As for why the GNOME variations seem to be prevalent? It's because some form of GNOME desktop was included as the default for the first widely popular "works out of the box" distros - Ubuntu, and Mint. the Son of Ubuntu.
People didn't install Ubuntu/Mint because of GNOME; they installed GNOME because it came along with Ubuntu/Mint. And 95% of those Linux users won't muck about and try different desktop systems because what they have just works.