Most languages can access C without much hassle (Ruby, Python, ...), F77 predates this, and while you can combine C and F77 for a particular compiler, it is a painful and unrewarding task (and the result is compiler-specific). It is often quicker to run the F77 though f2c so you have an entirely C codebase (even if f2c is not really readable).

Typically f2c code will be slower than F77.