Canonical showed up to the party with a substantial concerted and sustained effort of funding/developers to take the existing Debian framework and make it more user-friendly.
In particular, Ubuntu was willing to package substantially non-free software into the experience. Things like Flash just worked most of the time. There was a substantial emphasis on usability, so that Linux-on-the-desktop could reach people with less background knowledge (everyone was doing this, of course, but Ubuntu seemed to put less emphasis on other things).
When Ubuntu emerged, Debian users were concerned that Ubuntu would take over the DFSG-software world and make it, in the long run, less free. What turned out to happen, in this case, was that the approachability of Ubuntu, built atop the Debian foundation, drew many more users away from RedHat/Fedora and ultimately strengthened the Debian/Ubuntu userbase and ecosystem. The usability really mattered -- I've repaired the occasional Debian system with Ubuntu boot-disks simply because the drivers, at the time, worked better.
Thanks to the intrinsic properties of open-source development, many of the good things about Ubuntu have found their way into Debian (and GNU/Linux at large). The Debian of today is a much more-readily usable experience than the Debian of 2004 thanks to the work of a great many people.