"[Mate is] transitioning to GTK3 and refining the interface, slowly evolving things, fixing bugs, keeping things that make sense and removing things the user-base disagrees with."
In other words, it's converging with Cinnamon. MATE was about continuing GNOME 2 after the GNOME devs dropped it and decided that GNOME 3 was going to use a "converged" UI, while Cinnamon was about using GNOME 3 and its improvements over GNOME 2 but taking it more in a GNOME 2 direction in look and feel.
If MATE moves to GNOME 3, why even have it anymore? Y'know, once I wrote that, it occurs to me that I read (I think) about that possibility somewhere before (that the two would possibly merge into one project). FWIW, I tried both MATE and Cinnamon and found them to be quite similar, but Cinnamon was the one that I liked best. I liked the Nemo file manager more than Caja.