Surprised how many answers here are on "social" reasons, and ignoring that Ubuntu does provide genuine additions to Debian.
Ubuntu includes additional proprietary drivers to Debian. So there are legitimate hardware cases were Ubuntu will work out the box and Debian won't. I can't speak for how many people need those drivers, and how much of choice is down to defaults/inertia etc.
From hearing interviews with PopOS folk, I think this is also a big reason people choose Ubuntu as a base for a lot of Distros instead of Debian.