Right now, upstream (MATE) developers are working to get MATE 1.14 ready and it's being scheduled to be released in March to follow Ubuntu and Fedora's cycle. For 1.14, they are focusing on better support for GTK+3 toolkit and in order to do that, they will set a minimum version of GTK+3 supported for all build since GTK+3 changes a lot in every stable releases. You can see their roadmap to see what's their focus at the moment.
Since it's quite a big change for MATE and we are so close to Slackware 14.2, i decided to focus on 1.12 release for now until 14.2 gets released and then i can start working on supporting MATE 1.14. Since MATE 1.14 might contain mixed release (GTK+2 and GTK+3), i need more time to make sure the transition works nicely.
Personally i prefer not to provide a mixed release since it will be confusing and may lead to bad user experience, but we also have problem if we want to fully switch to GTK+3 build only. There are some parts that still have few critical bugs for GTK+3 and i prefer not to ship them for our users.