Why salt AND pepper and not salt and/or pepper
Pepper is likely just lucky to be a very commonly used spice that gets to hang with Salt all the time, though Salt really makes everyone else look good anyway.  Pepper just doesn't look good on its own.

Uh... Yeah.

I say you do see salt paired with plenty of other spices, but because "Salt and Pepper" is pretty engrained in our culture (refer to the potential duplicate thread), you don't really take note. Therefore, you will always see "Salt and X" as a matter of fact but rarely do you see "X" alone without that ever so essential Salt.

This answer's getting pretty close to the Answer in the other Salt and Pepper question, huh?