Most of Linux is written in it. Lot's of libraries are written in it (e.g. Python calls lots of C libraries, since it's just too slow to do things naively). It's simple. And while we are slowly getting real alternatives with Zig and Rust, it was for a long while one of the few practical compile languages, the other being C++, but that's an gigantic ugly monster of a language, which is why many people wanted to avoid that.