I assume it is not a technical issue, and that the profile UserId (numeric) is what is used as a primary key in the underlying supportive forum database (and not the username itself). Besides, SMF supports it, be it for oneself or for a third-party (I figure performed by Admin): https://wiki.simplemachines.org/smf/How_to_change_your_username

Being not a technical issue, what’s left is a logical reason. The one I can think of as plausible is that forum accounts here have a certain tie to finance. There are sellers, scammers, people you trust and people whom you distrust. Having people being able to change their username freely could easily lead to confusion as to whom you are dealing with. In a worst-case scenario, a trusted forum member could change his username, and have the old username taken by a scammer.