It could be a budget issue. SLoP is too far down the pipeline at USH and has become too expensive of an attraction. But, USH is stuck with it or they risk losing a lot of prep money. Yet, IoA hasn't started the ride yet, so they can get something cheaper to compensate.
At some point, USH may be getting the bulk of Comcast's budget to help them compete better with Disneyland. Sadly enough, it's a lot easier to compete with Disney World at this point...