The final assumption here is that Ubuntu's international embrace gives it the biggest lead of all. The above-mentioned Linux distributions are very much U.S.-based in their target user base. In recent years it has come out that language packs are available, but generally speaking there is something unifying about how Ubuntu presents itself to the world at large.
Maybe its due to the fact that it's not based here in the States and targets other regions actively? Needless to say there are a number of schools of thought on this.
Open community vs closed community distributions
Now here is where things get interesting. Ubuntu is, by and large, considered to be an "open community distribution" of Linux. This means that theres opportunity for newcomers to join in and contribute to the project where needed. Simply Mepis and PCLinuxOS are considered to be, by some people, "closed community distributions" because of who is involved with the direct development.
Understand this is not saying anything negative or positive about either approach. Rather Im pointing out their apparent differences and how they may be playing a part in why Ubuntu is able to maintain its hold over other Linux solutions.
Remember that in theory, anyone can "contribute" to the Ubuntu project at some level. So there is immediately a vested interest for all who choose to participate with the Ubuntu project.
Software and services
Based on Debian, Ubuntu has more access to software packages than many other decent Linux distributions available. This is not calling into question a matter of quality, rather pointing out that availability of the latest applications does matter to a great many users. Stuff like this colors their perception of desktop choices.
The fact is that Debian users enjoy tremendous software access. Ubuntu users arent strangers to this side benefit of running a distribution based on Debian's efforts. Where Ubuntu differs with regard to software, however, is that Ubuntu's applications may be tweaked variants of Debian-compatible software from what is known as "Debian Unstable." Not in all cases, but in enough instances to warrant concern for some people.
Regardless of the potential for headache, many Ubuntu enthusiasts have come to expect the latest and greatest advancements even with their perceived bug-related risks. And sometimes this means using buggy application releases that are known to have issues on the Ubuntu desktop. This might be no problem for advanced Ubuntu users, yet it's unfortunate that not all Ubuntu users are aware of this coming out of the gate.
Then there are the newly formed services that Ubuntu's main supporter Canonical is presenting to the world. Canonical's new "cloud" services for file storage and music management have proven to be a strong concepts. They ensure that Ubuntu remains in the headlines even when things may still be very much under heavy development and not quite as end-user ready as Canonical wants you to think. Ubuntus music service, for instance, has a long way to go in my opinion.
Going forward for competing distributions
By now it should be clear that we don't, in fact, live in an Ubuntu-only Linux community. Companies like Red Hat and IBM, among countless others, have made contributions along side of Canonical to help ensure that Linux for the masses is a great experience for all enthusiasts.
And while I accept that Simply Mepis and PCLinuxOS have little to no interest in taking Ubuntu head on, I would like to see more OEM adoption effort coming from the Fedora project and perhaps even OpenSuSE. Out of the two, I have seen Fedora come out swinging fairly hard with each of the last three releases. These releases are clearly supported by dedicated, hardcore developers who honestly care deeply about the product theyre releasing.
The latest release of Fedora is surprisingly excellent. Despite not being a big fan of RPM-based distributions in years past, Fedora changed my view completely -- it seems to work well without much trouble at all.