This interesting piece at xyzcomputing investigates why Ubuntu has become so popular, so fast!
Read the analysis here.
One very interesting aspect of Linux on the desktop is the popularity of each different distribution. Each version has a number of fundamental similarities with the others, but certain changes are made to each in order to differentiate them from the others and to enable that version to fulfill the niche for which it was designed. These differences are not always tremendous, but they are enough to allow people to make a definitive choice between one version and next. This makes much sense as the distribution system has allowed for the evolution of very specialized versions of Linux, not all of which are suitable for desktop use, just as it has been slowly perfecting Linux on the desktop.