The main factors are that it's a statically-typed language and that's compiled to machine code. Also, since it's a low-level language, it generally doesn't do anything you don't tell it to.
These are some other factors that come to mind.
- Variables are not automatically initialized
- No bounds checking on arrays
- Unchecked pointer manipulation
- No integer overflow checking
- Statically-typed variables
- Function calls are static (unless you use function pointers)
- Compiler writers have had lots of time to improve the optimizing code. Also, people program in C for the purpose of getting the best performance, so there's pressure to optimize the code.
- Parts of the language specification are implementation-defined, so compilers are free to do things in the most optimal way
Most static-typed languages could be compiled just as fast or faster than C though, especially if they can make assumptions that C can't because of pointer aliasing, etc.
