By then I had used up enough time not answering the question, and in the process I remembered something someone once told me. So I finished up by speculating that hinnies are less common than mules because they can be less predictable in both conformation and temperament. I was not happy with that answer, so the subject remained on my mind. Since then I've been collecting reasons why hinnies, and especially draft hinnies, are less common than mules. Not all of the following reasons make any more sense to me than the reason I rattled off over the radio.
More mares exist than jennets, and more jacks exist than stallions.
Since there are fewer jennets than mares, the gene pool is more limited. Jennets are smaller than mares, and since the foal grows to fit the size of the dam's uterus, hinnies tend to be smaller and lighter than mules. Good mares are cheaper than good Mammoth jennets. Since Mammoth jackstock is classified as endangered, most breeders use the jennets to reproduce their own kind, rather than to produce sterile hybrid mules. Jennets are less fertile than mares, which are not all that fertile to begin with.