Paranormal Activity must be one of the least R feeling R-rated franchises of all time. There's almost no explicit violence, there's not been any nudity that I can recall, and even the sexuality on display is tamer than what you might see on TV at 8pm. The MPAA sometimes gives horror films harsh ratings for being effective - ie, scary - but Paranormal Activity 3 earned its R for 'some violence, language, brief sexuality and drug use.' I have to assume that it's pot smoking and language that really earned the film the R (although I can't recall specific 'fucks' in the film there probably were a couple - school me on this if I'm wrong).
I believe it's the very soft R aspect of the series that makes it so popular; harder R horror films tend to appeal more to men, while movies like Paranormal Activity and The Ring and other 'scary' movies (as opposed to gory movies) crossover to female audiences. This isn't science, and it's not 100% true - there are plenty of women who want their horror hardcore. The Saw films didn't become such earners with just men in the audience. But generally these movies operate on the haunted house instead of the gross out principle, and that makes them safer for a wider audience.
Still, at an R-rating, these films aren't hitting the widest possible audience. These are Friday night movies, perfect for 15 year olds (who are obviously seeing them anyway, but are they buying tickets for something else and sneaking in?). That the Paranormal Activity films have gotten to a third movie and remain R-rated is actually baffling to me.
If the audience is getting a little bit sick of the Paranormal Activity formula, as that CinemaScore could indicate, going to PG-13 would allow Paramount to get new paying audiences in the door. And honestly, would anybody notice?