So back when this project was just getting started, we needed a name. We discovered 2 things, there had been several failed/aborted “Mubuntu” projects and Canonical now protects the *ubuntu names.
You’ll notice that only the early flavours enjoy a *ubuntu name, while newer flavours are named Ubuntu GNOME, Ubuntu MATE and Ubuntu Studio. On reflection, I prefer Ubuntu MATE and while I could request the Mubuntu name, I have no intention of doing so. Ubuntu MATE is well known now and changing the name would cause confusion.
As a little aside, when Ubuntu MATE got started we did contact all the other Mubuntus and invite them to participate. A few did but soon drifted off.