The "problem" with Mate is that it just works. I interact with Windows and OSX daily, and I have yet to find a feature that makes me thing "I wish my desktop environment could do this". Mate gets out of my way and lets me work.
In other words, it's boring. There's nothing new and shiny to be excited about, because it's as good as it gets. I left Gnome 3 because I was tired of visual effects getting on my way, but those visual effects were what attracted me to Gnome 3 to begin with. I think Mate lacks that "wow!" factor and, as grateful as I am for that, I think this might stop it from being more popular.