- Edited
h3o
I suspect this quote has a lot to do with it
I came from Ubuntu Studio on 14.04 which was one of the most polished XFCE experiences I've had to date. I was enjoying the low memory footprint and speed relative to Unity. Towards the end, XFCE had grown so buggy with other updates that I was forced to install a PPA to get a newer version just to have fewer issues. Going over 4 years without a new version was getting pretty bad. Then I installed Solus, and even with Budgie which was heavier on memory, everything was just so much faster and responsive on Solus that it no longer mattered to me the Budgie used a bit more RAM than I was used to. I have also used MATE on both systems and definitely think it's smoother on Solus.
I think most of us would say MATE is a superior experience because it has been actively maintained, seems to have fewer bugs, and feels the most like GNOME 2 these days.
Source: https://discuss.getsol.us/d/5092-about-the-mate-version/15
So there was a long time there where it was thought to be dead, xfce were still busy porting to gtk3 and Mate was much more active. 4 years without a release could be rather annoying for a rolling distro. Wanna update $X? We can't XFCE needs version x.y.z... Guess we'll wait a few more years for them to pull their finger out etc.