> I don't know why so many distros use Gnome by default.

There is a lot of history there. Back in the day, Linux and the open source BSDs had a plethora of different window managers and DEs. Everything from simple and old-fashioned MWM to the happy chaos of Enlightenment. By the late 90's KDE emerged from among all of this as a popular, if not dominant, choice. However, there was a serious problem. The Qt toolkit license was not GPL compatible. GNOME was founded, in part, as a true open source alternative to KDE.

Linux got big enough that the major distros felt the need to pick a standard DE. GNOME was solid by then, with no license issues by design, and there was a strong preference for GNOME among the open source thought leaders at the time. KDE had actually solved its license problem by then, but there were some strong feelings about the license controversy. So GNOME became the "standard."

But not really. SUSE, for instance, stuck with KDE.