At the end of the day if the race was in Feb or later there would be a lot more entrants. So many trainers give up on a prep because Dec races weren't standing up (or this year there were none around) so you're rushing your horse to be ready even though there will no other races for it, or people just don't even bother in the first place because it so early. I would love a dollar for how many times a Riverina trainer has said "I would have liked to get to the Breeders Plate but the horse just wasn't ready yet". If the race is in Feb or later that horse is ready and the heat fields have a lot more numbers.
Edit: Also, are they immune to heat in their paddock and on the float?