Anatomy! A cow has different anatomy that a horse. The cord that needs crimped in order to get the job done right is high up inside and can not be crimped with a band over time like a cow or sheep can. Some horses don't drop until they are over a year old and the scrotum is up tight next to the body, unlike a cow who's hang really low in the scrotum, as well as a sheep's. I imagine you need to do the banding when they are quite young, not possible for a horse. Putting that kinds of pressure on the testies would no doubt result in a massive infection instead of just a few dead fallen off nuts. Would be simpler and cheaper though hu.
The spermatic cord should be crushed and crimped, it is quite large and has a ample blood supply, done improperly and the horse could have a significant loose of blood. I have lost a horse to an improper gelding, not a good time, I tell ya.
