Final answer:
Rosemont emerged as the victor in the 1937 Santa Anita Handicap due to its exceptional performance and ability to close the gap on the other horses during the race.
Step-by-step explanation:
In the book "Seabiscuit," Rosemont emerged as the victor in the 1937 Santa Anita Handicap because of its exceptional performance in the race. The horse, trained to run at a fast pace, managed to close the gap on the other horses and crept closer and closer during the race. Wrangle, another horse in the race, gained on Jane Withersteen's legendary racers, which surprised the riders. Ultimately, Rosemont's speed and capabilities allowed it to emerge as the winner.