KDE is is designed to be customized at the cost of user friendliness. Cinnamon has a lot customization but it isn't as overwhelming and is much more stable. It also is GTK which is nice.
Because the Linux Mint team (I think it was on guy at that point) decided that KDE could not be adjusted to make the desktop they wanted. KDE isn't designed to be made into a separate system and neither is modern gnome. Gnome 2 was ideal and that is why it was forked and turned into cinnamon.
Linux Mint aims to be simple and easy to use. I don't believe KDE does that as well. The good news with KDE is that is is very customizable for those who like to tinker will the desktop.
I also think Linux Mint is heavily invested in GTK. Using KDE would involve a significant shift. What's wrong with KDE Neon?
it's not recommended, it causes massive issues with the built in programs and you will spend ore time fighting your system then just going with a native, I just tried this a few months back
I did install it via package manager back when I used this distro and it worked well, but some weeks after, I switched distros to Kubuntu. Now I'm using Arch btw. with latest KDE Plasma (I recommend this).