The shuttle operation is unique in the fact that it is easier to cancel a flight without disrupting the airplane's schedule for the day than it would be for a mainline flight. An example: Last month I was flying a shuttle trip and we got caught in one of the many snow storms to hit BOS. The airport temporarily ran out of deicing fluid (how can that happen in BOS?!) and it delayed us by a couple of hours. We were very late landing in DCA and arrived around the same time a later flight was going back to BOS. Instead of flying 2 flights up and back at the same time they made an operational decision to cancel our flight up and back. It was mid-day and both flights had light enough loads that they could both be accommodated on one airplane so why fly one airplane up and back full and one completely empty in both directions at the same time. That is about the only time it makes sense to cancel a round trip, any other time or circumstance would put the airplane and crew out of position for the remainder of the day. If they just started cancelling flights out of the blue on a clear, non weather day then the crews themselves would be in an uproar. If the engines aren't turning then the crews aren't getting paid and that would not be a popular option. I've flown some very lightly loaded flights from time to time, and there was never even a thought about cancelling them because of the load.
Also, cancelling a 3:00 or 4:00 shuttle is not very productive at all unless there are extreme circumstances, the flights at that time of day can run very heavy!