I have no idea, I'm afraid- the person who told me about this heard it themselves from a server tech person a while ago, and wasn't able to explain the reason behind it to me. I don't run a server myself, I just make maps for other people to use. Since I'm not really knowledgable on server-related things, someone would have to explain whether or not there's a custom map limit to me as though I'm a five year old.
Bump? I still don't have a very clear answer to this.