He told our stage manager that it was because it was too much to do on top of school... but the entire cast knows the real reason. He likes to party on the weekend, and because we have to be at the park from 5 pm to midnight on Friday, Saturday, and Sunday, his available time to party was diminished. He used to come into 9 am rehearsals telling us how he'd been out until 6 am the night before at random parties.
Even though it is a bit exhausting, it's actually a really fun thing to do, and the pay isn't half bad either.