Why? Well, physical perfection of any organism can be astonishing to watch. Horses are particularly gorgeous creatures, so a shot of a horse running as it was designed to run stirs the soul, just as an Olypmic athlete stirs the soul.
Add to it the mental aspect. The Triple Crown runs are chosen to be each different (the Preakness, a prolonged sprint - the Belmont a marathon), and the combined wiles of the jockey, the trainer, and yes, the horse, are fascinating.
Basically, it boils down to an inherent fascination in people and horses being really good at something. For all the shadiness and cruelty, that shines through sometimes.
Add to it the mental aspect. The Triple Crown runs are chosen to be each different (the Preakness, a prolonged sprint - the Belmont a marathon), and the combined wiles of the jockey, the trainer, and yes, the horse, are fascinating.
Basically, it boils down to an inherent fascination in people and horses being really good at something. For all the shadiness and cruelty, that shines through sometimes.