So why would they cancel an 8 PM flight on the morning of the flight then? I can't come up with any other explanation. As for people who were more than 3 hours away, there were other afternoon flights, just not non-stop ones, and they would have gotten us home just as late as the original flight. No matter what other flight we took, I could not use the last park day on my 4 day ticket.
Perhaps the 8 AM flight had mechanical issues. Keep in mind that an 8PM flight is potentially the 3rd or 4th (or more) for that plane in a given day. So if a early AM flight had issues, that would ripple down the chain. So that plane was supposed to go from A to B to C to D (where you would have boarded). But if it went down in A, they now had to find a new plane for B, and for C, and possibly for D. If they had nothing spare in the region, they may have had no choice but to look at available flights and figure out what cancellation had the least impact down the road.