I find it rather crazy that "cockpit", an almost brand new project has its entire backend written in C. Not your typical backend language by any means...

Well it does make deploymeny really simple, since it'll have such light dependencies. If it's statically linked it could even be as simple as unpacking a tar file.

Go can also compile to a static binary. Haskell almost (libgmp is the only hard dependency).

Go is less useful on older systems, since you have to go out and get the Go compiler/etc. and compile them yourself before you can compile anything else, whereas you can expect that all systems (not just recent ones) have a C compiler installed.

What's wrong with compiling elsewhere and transporting the binary there?

Also, there are builds of the compiler that one can use, you're not usually forced to compile the compiler.

What prevents limbmp from being statically linked in?

Licensing of libgmp does.