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.