I’ve seen people suggest Ubuntu without mentioning Kubuntu, but it seems like most people (especially gamers?) prefer KDE Plasma. I’ve seen people complain about Ubuntu’s GUI and lack of customization, but then no one suggests them to try Kubuntu instead. It seems like people just don’t know that Kubuntu exists.
Also as an aside, people often criticize how slow updates are on Ubuntu/Kubuntu/etc, but if you enable the backports ppa then it’s actually pretty quick!
sudo add-apt-repository ppa:kubuntu-ppa/backports
We could ask why not any other *buntu distro. Ubuntu is simply the default / most popular / most well known. Also, comparing desktops, Ubuntu seems modern (UI / UX) while KDE kinda gives me Windows 2000 vibes. I really prefer nice UI so KDE is just not pleasent to (my) eye.