The reason is this, the best interest for both Apple and AT&T, is to release "features" bit by bit. Because the iPhone is functional and scalable, and provides so many different type of users with so many options, by not including MMS in the plan, they not only create a "I want" mentality and it produces A LOT of buzz for them, this is just good
PR and Marketing, this tactic maximizes profits without blowing it all in one fiscal quarter. In return, when they do release the MMS option, they will have a whole "huge" demographic that will come buy the phone.