Because Air Wisconsin is leaving in a few months and they have 60 plus airplanes flying for Eagle.
Because PSA has SAP and 25%, (WAG) of the pilots fly between 65 and 68 hours a month.
Because AA already ordered them and are nearing their scheduled delivery and PSA can't accommodate them since they're maxed out at 1 per month form Envoy.
Because AA knows Skywest can better utilize a 40 million dollar asset with their work rules and keep the plane in the air more hours per day.
Because Skywest put a cheap bid in to keep the 700s productive since UA didn't want them any more. We never really know the details UA could still have some cost in those 700's since they were terminated early I believe.
I really don't know, just business thoughts I guess. I'd really just say there scared to death about the pending Wisconsin flying getting covered smoothly.

The original XJT, and SKW AA flying was before US and AA merger. Mostly breaking up Envoy's cost to cost operation.
The XJT AA 700's we're a 2 year deal ending at the end of 2018, the 8 extra sound like there attached to that deal and it hadent been extended. So it seems XJT will help wind down Wisconsin before they're in turn wound down.

I think the Director of Flight Operations there was a new hire FO in Fall 07 at his first airline.