Because the book isn't ready.
The author signs a contract to deliver the book by a certain date. As that date approaches, the publisher queries to see if the book is still on track. If the author states yes, then the publisher begins the pre-publication advertising and soliciting pre-orders. Then the book is put on sale and pre-orders are shipped. Publishers program books by year, 6 months, or 3 months based on their print capacity.
This book has seen several slips in it publication date due to either the author not having finished or issues in preparing the book for printing, etc. I feel that it finally slipped too far and would mess up the next planned printing block, or the author stated he needed significantly more time to finish.
Bottomline we don't know. With such a big jump, I assume the publisher has other books ready to go and slipped this one back as it still isn't ready for printing. It likely hasn't be send to the typesetters, so the publisher is moving ahead with other books at this time.
Just a guess.
