Up to now a significant amount of open source projects are all coded in c. In part, it's because a c compiler exists for almost any platform, and the small number of features the language has available meaning that it is easier to contribute to projects. The linux kernel being in c probably also leads by example.

I think the best thing to do though is to find a project you want to contribute to for personal reasons (you use it often for example) and try to learn the language according to that project. Often projects have style guidelines which means you could learn to code the language using the style that project is interested in. While you are learning the language you could ask contributors if there is any help you could do, sometimes just reproducing bugs and trying to provide useful bug reports to maintainers could get you involved in the project while you are learning to code.