You know that a kick to the head from a horse is generally lethal, right? You know that approaching a horse from behind always results in a kick to the head, right?
15 to 35 pounds of crap a day per horse. Thousand pound horse corpse being a biohazard. Fall from a horse, always serious. 5 to 12 gallons of water a day. Feed, shelter, know any blacksmiths? The infrastructure is gone, and there are various reasons why people willingly moved on from the horse.
I believe Brooklyn once a had a 10 acre lot that was piled to 15 stories high with horse droppings. Apparently, the flies blotted out the sun.