I can name two: simplicity and portability.
C is much simpler than C++, which is the closest thing to C among the OO languages. Just compare the K&R book to Stroustrup's "The C++ Programming Language". Don't get me wrong, I love C++. But it is a very complicated language, with an enormous number of language features, which take a long time to master. C, by comparison, is very simple with only a few constructs. And I am not talking about libraries here, just the bare language features themselves. The same is true if you compare C with Java, C#, or any other OO language. IMHO, the only language simpler than C is Scheme, a functional language.
The second reason is that there is a C compiler for everything. Every processor and every operating system. And I am not just talking about Windows vs. Linux. There are hundreds of embedded platforms, processors, and DSPs. You would be extremely lucky to have a C++ compiler for one of those, let alone a JVM. But you would definitely expect to have a C compiler.