A lot of times, people wake up, see that it's raining and then decide they don't want to go out. Being able to cancel the night before doesn't really help in these situations if a person's intention was to go to the parks but they only decided not to after seeing heavy enough rain.
I never cancelled a Disney park reservation based on the weather forecast the night before but I have decided not to show up at Hong Kong
Disneyland due to the weather when I already had a reservation.