The owners are looking at so much breeding money ($bajillions) from the two wins already under their belts they aren’t going to risk it with another race. If the horse comes up lame and gets the farewell gunshot to the head, that’s $bajillions flushed down the toilet. The owners are protecting themselves and cashing in before that can happen.
I do find it odd that we had several Triple Crown winners in the 70s, starting with Secretariat, and then after 1978, nothing.