A few things to note:
1= Those (entirely subjective) ratings have nothing to do with whether or not the entries in question were good Iron DM entries. The Anthology was compiled as a resource to DMs looking to use those adventures and the ratings reflect that purpose only.
2= Most of those entries did not have a specific word-count limit.
3= Most of those entries had a 24 hour time-limit. Given a 48 hour time-limit, entrants need some kind of challenging parameter to maintain the integrity of the IRON DM Tournament. The word-count limit fulfills this imperative quite nicely.