Then Ubuntu came out (well, before KDE 4, but during the funny licensing thing), which used Gnome because Debian used Gnome. Debian used Gnome because Qt had the QPL licensing: https://wiki.debian.org/DFSGLicenses#Q_Public_License_.28QPL...
Ubuntu got popular super quick, which got the DE popular, and the entire community just sort of got behind it.
If you're creating productivity tools for your customers, you probably don't want to channel money into multiple projects that compete with each other. So, it's not too surprising that RH wants to focus on just Gnome.