Because C is the descendant of Fortran/Algol68, not Pascal. While Algol68 used the := operator for assignment and = for equality, C followed the more familiar Fortran syntax, which used = for assignment and == for equality.
??? Algol68 yes. But Fortran???? C .NEQ. FORTRAN!
Just nodding in. Crack's a yawner. Wake me when it gets a debugger.