Because that would be a terrible idea. It’s too restrictive, and too many other distributions are doing that already.
Besides, Manjaro has only three official editions, i.e. …
- XFCE (the original Manjaro desktop)
- Plasma
- GNOME
All other desktop editions are community editions, developed by individual team members in their own time.
On top of that, there are reasons as to why Fedora focuses on GNOME, namely that GNOME is developed in-house at RedHat, and Fedora is a RedHat-sponsored project; it’s the “Testing” release for the next RHEL, and RHEL is a commercial product ─ it comes with a support contract ─ for corporate environments.
By the way, Fedora does support other desktop environments, so long as it isn’t Plasma. There’s a history behind that too; RedHat has in the past boycotted KDE, and has always treated KDE like a stepchild. So it was kind of to be expected that they would stop supporting it.
As for Ubuntu, the original Ubuntu did start off with GNOME only ─ the reason being that they wanted to pack as much software into their .iso as possible. But Canonical was and still is also sponsoring Kubuntu (KDE) and Xubuntu (XFCE), and in the past (and possibly still) Lubuntu (LXDE).
Canonical did try with some “innovations” of their own ─ the Unity desktop, which was just a modified GNOME, and Mir, as an alternative to Wayland. Both endeavors have failed. Mark Shuttleworth has also always been rather impulsive and whimsical ─ not exactly someone with a steady vision for the future.