Ubuntu derivative

Is one reason.

DE nobody else seems to be using

Cinnamon is easy to use though. Seamless transition from windows to linux for people who don't know what they are really doing. When they get the hang of it, you can do some neat stuff with it.

Cinnamon is also an in house thing from the Linux Mint developers which is why it's most common there. There's a few other distros that have spins on it. Namely Ubuntu, Manjaro, Arch, Fedora, etc.