If you did not get promoted, and you wonder why, when others of apparently comparable qualifications were promoted in the recent past, keep in mind the following:

1. Did you deliver on what was expected, indicated in your third-year review, or in conversations with your dean. If you did not deliver, you are already in a weak position.
2. While you might have reason to believe that prejudice was involved (race, gender, field of research, method, ...), if you did not do #1, you are in a weak position.
3. Have standards for performance risen in the last 5-6 years, say by the fact that your department or university has made appointments or promotions that are much stronger in that period.

If you have done what was expected, then you have reason to be concerned. Was the quality high, in terms of venues of publication, reviews, and the letters. Were the letter-writers authoritative--who they are, did they read the work, are they arm's length, giving detailed consideration of your work (which they have read!).

That you are beloved by your students, or valued by other units in the university, or ..., is rarely probative.

Nowadays, most schools and administrations are careful to be sure that matters of bias or unfairness are avoided. I don't know how successful they are.

As I keep saying, Living well is the best revenge. You really don't want to join a club that won't have you as a member.