I think it was the inability for GUI's to stay stable. Gnome went off the rails, Unity completely ruined Ubuntu[1], and KDE always felt very cheap and hard to navigate. My opinion, yes, but I always felt the GUI changes in Linux were extremely gratuitous after Gnome 2. Couple this with the fact that for a decade, the actual GUI was tied very, very heavily to the apps. So you had to use Gedit in Gnome, or whatever the…
Completely agree, and as a user of Desktop Linux for a very long time - I always felt that Linux GUI have been inferior to the options available on Mac and Windows.
It's not for everyone, but I found a personal solution with tiling window managers (Xmonad, I3) and related tools (suckless' dmenu). They restore sanity by providing lightweight, shortcut focused GUI systems, that don't intrude but provide enough functionality to get things done.
For anyone searching and testing GUIs like XFCE, don't dismiss tiling window managers as being too complicated or a bridge too far. My experience is that they work wonderfully in practice.