Seriously, "the Linux desktop is too fragmented" is bullshit.
You're not wrong, but it's definitely a contributing factor. The average user can basically just run out get get Windows, and for better or worse they know what they're getting. To them, Windows is Windows is Windows.
But it's more difficult for them to "run out and get Linux", partly because it's like the toothpaste aisle at the supermarket- 137 different options and they *genuinely* don't know what the differences are or why it would matter. They're afraid of making the 'wrong' choice, and I get it- I was there once.
I think a lot of folks look into Linux, get overwhelmed with all the choices and either freeze or go into "analysis paralysis" trying to figure which distro is right for them. (Most of the different flavors would be fine for them, but they don't know that.) And for a lot of people, I suspect that's where they stop.