the forums is designed in a way that 1) it's really easy to read threads without clutter, and 2) users are always associated with and/or responsible for their posts. this is to prevent general starfishry followed by a quick namechange so people who dont take the time to check dont realize this guy is the starfish who spammed the forums before, for example.
badspot also mentioned before he is against adding functionality to the site that would either increase the costs associated with hosting it (hosting images, for example) or clutter the reading flow. a list of old names would make profile pages cluttered over time, even with a time delay preventing you from constantly changing your name.
its not a terrible idea and its implementable in a way that avoids the issues above, but its not like theres much demand for it other than from people who want to hide their identity.