> This is the common refrain, but I think it's equally likely that it boils down to "I did this, so you should have to do this, too."

The reason is probably irrelevant: the organisation tells you what steps to take to get a promotion. If you fail to take those steps they consider you unsuitable for the promotion, not because they consider those steps to prove your capability, but because you have demonstrated an unwillingness to meet the minimum requirements.

Why the minimum requirements are what they are is irrelevant.