From a reader:

There is exactly one already-released book in the Amazon top 50 that is currently out of stock. Now, I would actually be surprised to find out that Amazon intentionally under-ordered the book. I suspect there is, in fact, genuine surprise among the powers that be about its tremendous appeal. Even assuming the most charitable interpretation, however, it probably does say something about the insularity of the book publishing/sales industry that the one and only bestselling book they failed to correctly forecast is a conservative one.