The original FRIDAY THE 13TH is my favorite movie of all time, and Betsy Palmer is a major reason why. That said, if I were trying to sell a remake/reimagining/whatever of the film to today's audiences, I wouldn't way to myself (or the studio bosses), "We have to make sure that Jason, the globally recognized face of the franchise, isn't really the killer!" As great as the original F13 is, if you tried to reboot this franchise with Mrs. Voorhees as the killer, it would undoubtedly be a financial failure. The general populace - the people who will ultimately determine whether this movie is a hit or a miss at the box-office - want and expect Jason. If you made it clear up front that Jason isn't the killer, moviegoers would stay away in droves. If you tried to keep it a secret and reveal it at the end, ala the original film, a majority of viewers would be decidedly underwhelmed, and perhaps even feel cheated (because they expected it to be Jason).
Sometimes I wish they would teach classes on the film industry and marketing in junior high school.