It's mostly because Universal's decisions, like this one, causes the results to be self fulfilling. They never give any of these shows, and late evening closings, a chance to be impregnated in guests minds/habits. That Six Flags analogy might have made some sense when Universal only had three hotels, but it makes very poor business sense with all the on site hotels filled with guests that are focused to on site activities.
And, keep in mind, March, as I said above, is prime tourist season. No reason not to run the shows in March with the Spring Break crowds.