Dear Michel,
The Ubuntu people work with Local Community Teams to support and promote the distro.
I think Ubuntu has a totally different approach. CentOS is a distribution targeted to the enterprise market. (Of course even your grandma can use it for daily business).
But when you notice the discussions on the MLs and the Forums, they are highly technical.
Ubuntu is more a social network. You are even welcome on most Ubuntu forums if you ask totally newb questions and will receive a friendly answer if you'll asks exactly the same dump question a few days later, again.
This is why Ubuntu is accpeted by linux newcomers.
I guess there aren't even so many CentOS contributors around (as Ralph said) that are willed to spend their time on promotion (maybe I am wrong).
Best Regards Marcus