Why do movie trailers reveal everything?
Why did Fringe reveal that Leonard Nimoy was in the episode, so that you are expecting him at the pivotal moment at the end?
Why do TV showrunners let people know what’s going to happen to characters in advance?
Do we need to be told what is going to happen so that we watch?
Why can’t there be more mystery and surprise with modern media?
Some examples of shows and movies doing it right: The Leftovers on HBO. I have no idea where that show is going. It weekly surprises me and keeps me coming back, hoping for answers. Star Wars VII: The Force Awakens announced an enormous cast and has left many out of the trailers including Luke Skywalker (supposedly, there are a lot of wild fan theories).
Think about your own marketing. Are you creating room for surprise and mystery or are you just telling people what to expect?