I don't know about the CRJ, maybe because Skywest crews are cheaper and therefore they will keep hundreds of them instead? As for the ERJ, it's because Skywest negotiated a money losing CPA and for all they have done to capture performance incentives, it's still unprofitable. So unless they can convince UAL to pay more for those aircraft, it makes sense to shrink the ERJ fleet to the lowest contractual amount possible. That happens to be 101 aircraft right now.