The developers of Linux Mint develop the Cinnamon desktop, they are close and/or share some members with the MATE desktop team, and so Linux Mint is pretty much that. There are several other good distros for KDE Plasma including KDE’s own distro, Neon, so they figured they weren’t really serving much of a purpose with it. Plus Plasma is qt, MATE and Cinnamon (and xfce AFAIK) are all GTK, so.