They did it for the Christmas DAH.
The real reason is that they need to park cleared before the party, which means the park "closes" at 6 pm. They don't want to encourage day guests to stay in the park, which fireworks would do.
Based on last night they need to HARD CLOSE MK on party nights. No shopping, no food, and no lingering. It was an absolute nightmare trying to navigate between 5:45-7PM. So many folks trying to eat, shop, and just linger as absolutely long as possible because they didn’t have party tickets.