I asked my T about this today, to see her perspective of why some people have polyfragmented systems and others don't, since she was the one who described my system as polyfragmented. (Please note: I want to be very clear I am not saying it is a diagnosis, it is a description of a DID system that has a large number of alters, some of whom have an extremely limited sense of agency. My diagnosis is DID. I am sorry if my use of that term given to me by my T is upsetting to anyone).
Anyway, my T said in her experience people with polyfragmented systems tend to be highly creative and to have experienced ongoing trauma over an extended period of time. I have no clue where she got her information from though, but since the term is widely used and accepted I suppose it has just become a descriptor used by some Ts over time. My Ts thoughts on polyfragmented DID seem to match up with your thoughts on this Alwayschanging. It makes sense to me.