MATE Desktop And GTK3
The MATE desktop is a fork of GNOME 2. The fork was a result of users being dissatisfied with the development of GNOME 3 and GTK 3.
With the release of version 1.18, MATE no longer uses GTK2 and only uses GTK3.
The GNOME developers have a reputation of not embracing what other people want. They focus on their own agenda, which they have standing to do. The caveat is anybody dependent upon GTK3 is dependent upon the whims of GNOME developers.
Possibly the MATE developers no longer control their own destiny.