Never thought about this question. The author of the article still doesn’t give an answer to the question (if there is any), but it suggests some possible answers.
I think this is the answer I agree with the most.
Any conference that might be about C is instead focused on some particular subset of its application. It’s not a C conference, it’s a database conference, or an embedded conference, or a Linux conference, or a BSD conference, etc.
Because C is used in so many places, a conference about C would be so big that instead it is split into different conferences.
After reading this article I was thinking about programming language conferences. If I remember correctly most of the programming language conferences are not about the programming language itself, but instead, it’s about some frameworks/libraries/use of the programming language. I don’t remember any particular conference that is only about the language and nothing about the applications of the language.