Honest question, why is it that whenever any party tries making a new DE/WM it always turns out to be a GNOME derivative? Unity, Cinnamon, Budgie, Pantheon, MATE.. and now COSMIC. Why not choose something else like KDE or XFCE to fork?
Because mostly KDE and XFCE are very customisable with few configuration files doesn't require creating whole new DE
The other way of looking at it is that Gnome is rampantly opinionated, and if you disagree, a fork might be your best option far more often than it is with other desktop environments.
Budgie and Pantheon are built on the GTK toolkit but aren't forks of Gnome, similar to xfce.