I wouldn’t say this with 100% confidence, but Orlando has two parks, so opening the event before Labor Day means that IOA can still be open later for regular guests, who have no interest in HHN.
Starting the event earlier, here, would mean losing a lot of potential revenue from non HHN guests with shortened hours, in one of the busiest seasons for “daytime ops.”
I’m not sure the trade off would be worth it.
Just a theory, though.