> Why use C instead of D, Rust, Go, C#, Java, Perl, Python, Ruby, Scala, Clojure, Erlang, Elixir, Haskell, Swift, OCaml, Objective-C...?

Because C runs pretty much anywhere? There are plenty of platforms where C is available where I doubt you'd find any of the others above (e.g. C64; yes there are C compilers for them; yes, I'm mentioning it tongue in cheek)

Because you can generate small, compact static executables? E.g. I used to write network monitoring software and an accompanying SNMP server for a system with 4MB RAM and 4MB flash, the latter of which had to include the Linux kernel and a shell on top of the application in question. The system was so limited we did not run a normal init, and couldn't fit bash - instead we ended up running ash as the init...

There are plenty of use-cases where "web application" == "user interface for a tiny embedded platform".