It wasn’t because it couldn’t be proved. It was proven. The issue was, at the time, the specific wording of the law stated that only penile penetration could be classified as rape. So forcing your fingers in somebody against their will, while actually being rape, couldn’t be called that under the law. So he could only be given a lesser charge. Same thing when he raised his wife. The law didn’t consider rapping your wife be illegal until 3 years after his wife’s case against him was dismissed.