Hot dogs have a very short shelf life compared to burgers. If they are not consumed immediately, they begin to shrink and change color. Visually, they are unappealing. Hot dogs can't be dressed and wrapped like a burger, they begin to sweat and the bun becomes unpleasantly moist.
People are also more particular about their hot dog garnishes and having to individually dress each one will slow the service down at fast food places a lot.