Maybe because a "series" of books are not really a "variation" as it was intended.
Now a listing with a single book with a choice of hardcover or paper-bound or special edition would actually be a variation. The problem with that type of variation is that it would lead to Search Manipulation because the price range is usually quite significant.
Variation listings are intended for items that are the same other than size, color or quantity, not intended for items that are completely different other than the general category.
Paranoia strikes deep
Into your life it will creep