The name-changing previously did get rather ridiculous, and caused problems not only for our membership but for staff, which does try to keep a handle on members who are developing patterns of deceit or manipulation, etc. So it was code-enforced that the display name could not be changed except by an admin. The restriction to one-time-only was due to the fact that some people were changing their names multiple times in a week or, in extreme cases, in a day. Frankly, while every single member of the staff is eager and willing to help the members even with what might seem like trivialities, we are all volunteers and we do not have unlimited time. We're not unreasonable and definitely don't want to answer users with, "No! You're stuck with that goofy display name for.ev.er!" but we also wanted to make sure that weighted consideration was given to name change requests by the requestee. The method of ensuring that is to limit the number of possible changes to one.
So that's the reasoning.