I would bet it's an exlusitivity deal. There a studios that give exclusitivity rights for a period of time after it comes out either to rent or buy digitially. I have not heard anything specific but I bet that is it.
When it was first available of DVD and Blue-ray iT was on iTunes as well, so I purchased then. It was in my purchases and I could redownload it at will on my iPad but recently I checked and it was gone!