When asked what's going with Luke's lack of appearance in the Star Wars: The Force Awakens trailers, director J.J. Abrams stated it's part of the plan.
"These are good questions to be asking. I can’t wait for you to find out the answer," he said. The fact Luke is being kept away from the promotional materials is "no accident," he continued.
It actually goes a bit deeper than that. There was a leaked image of Luke Skywalker wearing what seemed to be standard Jedi robes that made the rounds, but Disney went to work pulling as many copies of the image from the internet as possible, including Twitter embeds. The social media discussions of the image all look like this:
It's clear that Luke is being held back from fans until the film is released and we can see him for ourselves, in whatever role he plays in the film's story. This is one of the best things about the marketing campaign and trailers for The Force Awakens: They all deal with tone and emotion way more than story beats.
We know the barest amount of information about these characters and how they'll interact, leading to endless speculation and discussion about even basic story ideas and relationships. Luke is no different in this regard, and as the primary protagonist of the original trilogy there's much value in keeping details about his role a secret for as long as possible.Star Wars: The Force Awakens teaser