If you're reading this, it may be because I have declined your request for a new username. (Or you could just be curious)
First off, the reasoning for declining your request is usually written in a comment after the 
Not done text. Normally, username change rejections are done because they violate the local username policy. See WP:Username policy.
Secondly, your request may have been declined because it violated the global rename policy. If your request involves being renamed to a username that is already registered, it cannot be processed in the regular venues. See m:SRUC for a steward to review your request.
Questions? Ask me.