The stories are not grouped in categories because some stories qualify in more than one category. For example, "Platonic" could be in best relationship/best drama/best overall, and some of the shorter stories could be best waff/best revelation/best comedy, etc.
It is probably a lot easier to just list the stories and let the voters pick the categories for which they want to nominate them. Assuming people look at the lengths, it shouldn't be too hard, and at least for me, I knew the stories that I knew, and it was pretty obvious which categories they fit into.
That's exactly what is done and why it's done this way. Many stories fit many categories, and the categories can all be fairly subjective. The only thing not subjective is the lengths.
