The reason is a practical one. A great deal of work and effort goes into these books and someone is then, hopefully, going to buy a copy and I want that person to enjoy the book. With the A4 format the photographs, illustrations, etc., come out in a lot more detail because, obviously, it is the larger format. I think the quality holds up in both so it depends on what the customer wants.
Personally, I think the larger format is best so I will stick to that in future.