Only Stable Upstream Releases Enter Debian
The MATE packaging team does only upload stable upstream releases of MATE to Debian unstable which is common practice for all other major desktops used in Debian (KDE, GNOME, XFCE).
The reason is that both Debian and any of the desktop environments have long release cycles and it is very difficult to coordinate the availability of the latest stable packages of a desktop in Debian when we're about to freeze.
Since desktops consist of a large number of packages which take some time to be packaged and migrated into testing, it is very important to always have a consistent and stable set of packages in testing such that when the freeze happens in Debian, the set of desktop packages is ready to be shipped in stable beside some minor bugs. Once we're frozen, it is NOT possible to upload new upstream versions.