As I understood it, it was, at least originally, an ABC/ESPN/Disney issue. As I heard about it, its been going on for a few yrs now, the parent company didnt have enough HD space to transmit the alternate game in HD for any provider, including Directv. Now whether that has now changed an Dish hasnt been able to jump on the wagon becuz of their dispute w/ ESPN/ABC/Disney remains to be seen.
I myself had to watch the USC game in SD also (via KGO TV A.A.D.), at least for most of the 1st half as our local ABC pre-empt the game for coverage of the storms that swept thru the mid-atlantic.