This doesn't create a relevant distinction between the groups, it just tells us that neither was especially motivated by money. We still don't know why the offer of prizes caused worse performance.

So the group who knew they were writing for prizes wrote unoriginal stories, whereas the other group wrote with relatively more creativity since they didn’t know they were competing. This explains the difference between the groups.

We need to know why one group wrote worse stories than the other. This lacks any comparative aspect, because both groups' topics were suggested by someone else.

This doesn't offer us any distinction between the groups. So what if the judges like realism? We have no reason to believe that either group was writing more realistically.

This doesn’t explain why one group wrote worse stories than the other. We need a comparative aspect in our answer, whereas this lack of explanation applies equally to both groups.