Probably bandwidth. You download a game or five and then you're good for a few weeks, whereas if you are streaming media you could run through several gigabytes a day of data per customer in perpetuity.
Obviously, with streaming media there is a continuously refreshing pool of money to cover those costs as compared to games being a one-time purchase, but even with that it would still take quite a while to expend the entire revenue of the purchased game in download expenses and storage overhead.