Debian is very solid and is constantly in evolution, every so often they freeze the development at a point and call it a release. There is no fixed timing on this they just make sure everything is right before calling it stable. Ubuntu and other distros piggy back off Debian but rather than focus on stability (IMHO) they focus on a fixed release schedule and making it more amenable to the general public. Debian is a b1tch to install - Ubuntu is a breeze. Now, if Mint have made Debian more accesible then that's great news. I've also been following AuroraOS which again has ditched Ubuntu as it's base and gone back to Debian.
I don't know the ins and outs of it, but I know a lot of folks seem to be unhappy at how Ubuntu is evolving - it's priorities etc..