You didn't ask me and it isn't my decision, but I think the logic of not allowing a change is twofold.
1) Why should someone be able to lock up a time that someone else might want only to change once they see what everyone else is doing?
2) The reason to go early is to lock up the time you want. That's the reward for getting ahead of the crowd. The risk is that you have less information to go on.
I imagine they would probably add that the amount of work and potential confusion could be staggering, but that is just speculation.