They want to maximize profit. Over many years they created the perception that Hardback is much more expensive to print than trade paperback and that is more expensive than mass market paperback. So, first they publish it as a hardback, for $50 and when they run out of people that are prepared to pay that they publish in cheaper format.

The thing is, the printing of the hardback is only a tiny bit more expensive than that large paperback, which is tiny bit more expensive than small paperback. Certainly the difference is much, much smaller than the retail price difference.

This has backfired hard, when they started to publish e-books and people expected that an e-book would be markedly cheaper than the cheapest paperback.