Why was Black Beauty stopped being used as a workhorse ?

Black Beauty stopped being used as a workhorse because he was overworked and exhausted. Rather than follow the farrier's instructions, the Skinner decided to get rid of Beauty.

“This is a case of overwork more than disease, and if you could give him a run off for six months he would be able to work again; but now there is not an ounce of strength left in him.”

“Then he must just go to the dogs,” said Skinner. “I have no meadows to nurse sick horses in—he might get well or he might not; that sort of thing don't suit my business; my plan is to work 'em as long as they'll go, and then sell 'em for what they'll fetch, at the knacker's or elsewhere.”