I don't see you answer what @OP asked, he's asking what the reason why we can't change our username for specific period of time. In this case, only the administrators can answer it because no one know the reason (except the administrators posted it before).
That is not advisable. Some people will abuse it.
@OP said the old username will stay, even someone changed their username, we can still know who were them in their custom title.
On the side of showing when the user changes their username, theymos made a statement that is against that; if someone for any reason changes their name, there is no need to make it too obvious that they did change it; it could be a security risk to their privacy.
Not all changed username for privacy reason.
Each user has a display name, which is the only name ordinary users see, and a username, which is what you use to log-in with. Almost always, they are the same.
If I change a name for reasons of appearance, then I only change the display name. The username remains reserved, the user can still login using the username, PMs can still be sent to that username, "search member" allows searching for the old username, etc.
If I change a name for reasons of privacy, then I change both the username and the display name. In this case, it is possibly hazardous to reserve the old username, since it allows someone to test for the existence of that username, possibly defeating the privacy benefit. If you're worried about being impersonated, then simply don't seek to have your username changed.