There are a few (good) reasons tbh aswell.
Number one for me is that it becomes confusing for new members trying to learn other people's names. By now everybody's probably noticed that very few of us actually call each other by their forum names (Cthulhu is Yop, Twilight Mexican is Tres, Cookie Monster is Road etc.). To alleviate this, everyone is required to put their old usernames in their "AKA" field and people who wanted to be trolls or simply didn't like rules just flat out refused to do it.
And we're talking about people who would change their usernames at least every week so they have AKA fields the size of an essay. Then at one point I think there was a rule that you could only change it a maximum of once every two weeks.
Since nobody's tested it for a while none of these things are really actively enforced, but it was so abused at one point I'd honestly feel uneasy just opening it up. A lot of forums don't allow username changes at all. I'm glad we do because being stuck with a username you don't want is shitty, but I'm fine with the way it currently is (although I don't change my username like ever) and tbh I think it could stand to be a little more strict XD, but that clashes with our general ethos of "meh".