They built up their user base many years ago by providing some level of resources and coordination of desktop usability built on top of Debian's unstable release. Thus becoming something of the default target for application developers who wanted to support Linux. Debian never had the polish that Ubuntu did with its catchy animal based nicknames and 6 month release cycle. They built everything on top of the Debian community and just provided a little bit more testing and marketing. Ubuntu released it's first release almost 20 years ago with Warty Warthog. I switched to Ubuntu around that time as my primary machine although I switched to POP! os by System76 which is a desktop distribution built on top of Ubuntu that focuses on the desktop when System76 engineers decided Ubuntu was straying from supporting desktop Linux.