Currently the upside to directly delivery is small; there just aren't that many people who have the hardware, bandwidth, and inclination to buy shows online. Certainly nothing like the 28 million subscribers they currently have.
There is no serious incentive for them to be a pioneer in this. Amazon, Apple, and Netflix are doing a fine job of figuring out what a la carte TV show delivery will eventually mean. When the market is big enough to make it worth getting in a fight with the cable monopolies, I'm sure HBO will do it. But for now, they can let other people take the risk.