Assuming that it will show up in iTunes (not all of them do), many studios maintain an exclusivity period of a couple of weeks to 30 days before before the movie gets distributed digitally to make sure that DVD sales are not (in their minds) affected negatively.
Maybe Apple deemed it so bad they couldn't put it up on iTunes. Wait that can't be true, Leprechaun 6: Back 2 tha' hood is in iTunes, so that theory is disproved....
On a serious note, I remember renting the first Transformers movie back in January of 08', but when I went to buy it in April, it was nowhere to be found. It's very odd.