According to link Head Boys and Girls are chosen from all the seventh years. Perhaps it's easier to be chosen if آپ were a Prefect before, یا maybe it was just a coincidence that all the Head Boys and Girls we know about were Prefects?
I'm pretty sure Remus was still a prefect. You're right, usually it's 2 prefects that become head girl and head boy. So I'm just guessing here, but I think James wasn't mature enough to be a prefect in their 5th year, and Remus obviously was. سے طرف کی their 7th year, he was, and James was probably مزید of a leader than Remus. Sorry, I'm just guessing here, someone else might give آپ a better answer.