[exceptione]

I think they have build up a large userbase over the years. It is pretty beginner friendly imho. The upside of it being arch based is that your favourite package is not years old. For a desktop os a rolling release where every imaginable package can be found in either the official repo or in aur is a huge plus.

Endeavouros came later, when Manjaro was already large and established. It has a more sane approach than Manjaro wrt package mgmt. But by not including a *graphical* package manager they are not an option for beginners. This is possibly meant to prevent success at any cost, I dunno.