I think it's because it's just well done and easy to use also their forum participation is excellent.Having the backing of a multimillionaire driving developement can't hurt either. Ubuntu does seem have more freedom than say Linspire or Xandros.
Also with Ubuntu's pact with Linspire each consentrating on enhancing 1 desktop both Gnome(Ubuntu) and KDE should really shine on those distros.. plus I think they are going to share access to those desktops so that Kubuntu will use Linspire's KDEand Linspire users will be able to install Ubuntu's Gnome.
Either way it's good news for linux in general..
Dell is thinking about boosting their sales by offering Linux pre-installed on thier comps.. add to that more distro's joining or thinking about joining
CNR.. If it becomes concidered a success look for more and more companies to offer a Linux version of their software through such a service. Linux really is on the verge of becoming a viable desktop solution for the masses.