Here's my guess:
I think there is still a slight excess in training capacity on the Bus. If they had a few extra Buses, they could send some of the new 737s to get modified with wifi/entertainment systems.
Once complete, they could keep the Buses, or park them.
Used airplanes, intended for short-time use, would be cheap to acquire, and have no long-term maintenance costs (ie, D-checks).
This would reduce the training footprint on the 737 as well.