Re: "These are beta releases"
14.04 is the current LTS release. Everything that comes out until the next LTS release is where the developers get to try out new stuff.
If I recall correctly, the overwhelming majority of Ubuntu users stick with the Unity version, and they upgrade from LTS release to LTS release and don't try out the intermediate versions or alternate desktops. These tend to be people who are more interested in just using their PC rather than experimenting with it.
The Ubuntu MATE users seem to be largely former Xubuntu users or people switching from Mint to Ubuntu because of problems with the former. Kubuntu has never been that popular with KDE fans, personally I think that other distros have tended to do KDE better. The target market for Lubuntu was old low end PCs, but with the original hardware targets dying of old age and even the lowest end of most of the "old PCs" still around today having more than enough oomph to run the more mainstream desktops, there's not a lot of reason for it anymore so fewer people are using it.
The company behind Ubuntu, Canonical, backs the version with Unity. The other desktops are put out by enthusiasts who get to use Canonical's servers for free.
Biting the hand that feeds IT
