In an interview with AP, Star Wars: The Force Awakens director J.J. Abrams admitted that Luke Skywalker’s physical absence from the trailers was intentional.
Fans have been speculating as to what it could mean for the plot. I was thinking earlier that it could be because Luke went into exile like Yoda. However, fan theories are buzzing that the true reason for Luke’s disappearance is because he turned to dark side of the Force. The theory is summed up nicely here by the HuffPost. It would be heartbreaking if that were the case, but at the same time would it be that surprising? After all, Luke did disobey Yoda when he went to save his friends and confront Darth Vader. Plus, as we learned in the prequel trilogy, Jedi are trained from a very young age and Luke was even older than his dad when he started his training. We’ll find out on December, 18th.