ESPN is owned by Disney[1], and I believe it is considered a channel conflict (i.e. don't compete with your customers). In effect, by making their content less scarce, their customers (Comcast, Dish) would ask for a discount during the negotiations because the content is now perceived as less valuable. In addition, advertisers simply won't pay the same rates to advertise against "internet content" as they will for cable or broadcast. My suspicion is that it has something to do with the potential for divided attention of the viewer.
[1]http://mediakit.go.com/espn/