Because a crappy sequel devalues the rest of the franchise, especially in the eyes of the unfortunates who see the shitfest before seeing the good stuff. For example, if I had managed to see "Jaws: The Revenge" before seeing the rest of the big-ass killer shark movies, I probably wouldn't have bothered with the original (which is probably one of my ten favorite movies) for several more years, based purely on the foul taste that movie left in my mouth.