Remember, the A320 is a 15-year-old design. If they have newer 734s and A32Xs, they would want to keep those around in favor of the older A32Xs.
It could be that they simply do not need these particular aircraft (i.e. at present, 737s run particular routes at greater efficiency or passenger preference or less cost in fees or whatever), but will reactivate them when they time comes that they need them.
Those who fail to learn history are doomed to repeat it in summer school.