Apparently, we love sequels. And you can almost be certain that if a movie becomes a big hit, then the producers will create one or more followups. This does not have to be a bad thing, but it starts to become silly when we are talking about 5 or more films (Saw, Paranormal Activity, Final Destination). I am uncertain why this trend has become so dominating. Perhaps is it connected to the fact that tv-series has become so successful and they want to mimic this over to films.
Anyway, the problem with serializing horror films is the same problem you get from showing the monster too many times: It stops being scary. The unknown will become known. In addition, the sequels follow the same structural development. In other words, we are watching the same film, with a slightly different packaging. The only reason to watch the films, is to pick up the several tiny hints of a bigger connection between them.