Blame it on the short attention span of the media
According to the Authors Guild Bulletin, there used to be a year between the launch of the hardback and the release of the paperback -- and that's the way I remember it, too.
Now, publishers, recognizing that the only attention a book is likely to receive is around the time of the appearance of the hardback, are either following more swiftly with the paperback edition, or forgetting about paperbacks altogether.
Some publishers still rely on paperbacks to keep a book in print, and compensate by bringing out the paperback quickly, while the reviewers are still paying attention.
Increasingly, however, publishers, having recognized that Kindle books are outselling paperbacks on Amazon, are happy to let a book exist only as an eBook, once the hardback is out of stock.