Ketchup and a little mayo are good, because they bring their own flavors to the party but they don't go around thinking they're all awesome and taking over.
Mustard and relish, on the other hand, bring more than their fair share. If you add mustard to the hot dog, every bite will taste mostly of mustard, only slightly of hot dog. Some people may respect that in a condiment. I don't. I'm sitting back in the corner with ketchup and a little mayo thinking, "Sheesh, who invited those guys?"
Somebody had to say it.