CentOS is actually a pretty popular distro. It is a rebuilt clone of Redhat Enterprise Linux, with the branding removed. It is mainly used as a server distro due to it's notorious stability. It is not as popular as a desktop distro however.
The stability is achieved by extensive testing and patching of software,. The long period of testing leads to the software being several versions outdated. I believe this would be the main reason CentOS is not as popular as a desktop distro. As one other point, because Redhat Enterprise Linux, and thus, CentOS too, are primarily built for server/office use, the repo lacks a lot of media software desirable by desktop users. This can deter some people too. There are third party repos for media software, but those packages haven't gone through the extensive testing the software in the main repo has, and sometimes can be incompatible with other third party repos often used.
These obstacles are quite avoidable under most other distros that cater for desktops. Why use CentOS at all then? because as i previously said, the stability makes it an excellent choice for the server if you don't want to pay for a Redhat license and don't need support. Other distros which cater more for the desktop provide more up to date software, providing more features, but usually with a sacrifice to the stability of the overall system. The stability of a distro, seems to vary between distos; usually with the more up to date software provided, the less stable it becomes.
Some distros believe in providing vanilla software, which means that the software is provided exactly (or as close as possible to) how the developers intended it to; not patching software at all (except perhaps security patches). Arch Linux and Slackware are two such distros. Most other distros create patches for some bugs before a piece of software is added to the repo. Distros that ship vanilla software provide software 'as is' because they think that additional patches added from distro devs can often lead to bugs in other parts. Because of additional patches added by distro devs, a official bug report can not be sent to the original devs of the software either, but instead needs to be sent to the distro devs. With distros that ship vanilla software, a bug report can be sent directly to the people whom created the software.
I hope that's helped; i've expanded a little on the original question, but i think it's all relevant and covers a few important points that are often over looked by Gnu/Linux users.