>I always wondered why?

for me personally easier to use installer, proprietary driver/codec support out of the box, faster release cycle, debian stable is too old for me and unstable can break at at all times/doesn't receive any major updates before new releases and so on. Also Ubuntu for the longest time used to be the only distro shipping with good font rendering settings, staring at code all day this actually was fairly important to me

so basically Ubuntu 'just works', if I ran Debian I'd just replicate Ubuntu's default settings pretty much anyway so I can just save myself the few hours.