The most likely reason you were not promoted is that another player snuck into your spot at the last minute. Fight Pit positions and promotions are all dependent upon others' rankings as well.
A less likely situation could be that you are stuck in a broken Fight Pit division. There is a slim chance that this happens, but it can happen. IF you are one of the unlucky few, please post the issue in UserVoice to help us identify whether this is a larger issue.