Talk to your employer. Let them know you want to know why you were passed over so that you can work on the problem areas. That way you know what to focus on and can get the promotion next time. Good luck!
I've asked myself this many times why!? Not me! They have there reasons, and it sucks. But sometimes they want you where you are because you are the best person for the job you are doing . Tell them you want more responsibility, if that doesn't work go over his head and talk to his supervisor and state your case. But beware heads might roll and yours might be one of them.