Actually, since Ubuntu has basically everything available that's available in Debian, it doesn't eliminate your option to choose something else, it's just a different set of defaults on a different release schedule.
These two things (a faster, predictable release schedule, and usable defaults) are what has made Ubuntu popular.
At the time Ubuntu started, getting a new desktop/GUI system running was a fairly significant amount of work (not as much work as some made it out to be, but work). After Ubuntu demonstrated how easy it should be, the other desktop distros have drastically improved. Unfortunantly, Debian has not gotten quite as good, some of it may be that Debian is aimed at a far wider range of uses than just the Desktop/GUI segment.