Because it is far and away the most feature complete, polished, and stable DE that exists in the Linux ecosystem. It requires zero learning curve, has an app suite that isn’t riddled with bugs, everything for the desktop user can be done in the GUI.
Like there is no alternative that does what GNOME does. It’s GNOME and forks of GNOME, a chasm, then KDE, another chasm, and then the DIY DEs/WMs dejour.