Well yeah we do have general conferences on programming languages. But the point is we don't have a specific conference on C, like you wouldn't have a specific conference on a simple hammer.
Lots of people learn and use C as an every day tool. I'm a Java and Ruby programmer - but I have to work with C as an ABI and an extension language. Python programmers have to work with C as an extension language. There are DB developers who use C.
I don't know anyone who describes themselves as a 'C programmer' like you would 'Ruby programmer'.