These guys do a good job of explaining the situation.
A quick summation.... an on track vet was the one that made the decision to originally scratch Modern Games, this after them being behind the gate and thinking that they saw the horse break through the gate and assuming it would be physically unable to race. It was shortly thereafter that they realized the front of the gate had been opened, by the gate crew and not by the horse, that they again contacted the stewards confirming that the horse was okay to run after being observed. Stewards make decision to let the #1 run for purse money only.
If this had been the first race on the card, and not one that looked like it could be run in diminishing daylight, they would have had more time to make a better judgement call. Perfect storm for chaos, black eye for the sport.