Eh, I'd say it's pretty likely that the "sold out" parties last year had different capacities. Here's the thrill-data average wait times by party:
View live and historical wait times for Mickey's Not So Scary Halloween Party at Magic Kingdom for 2023 on Thrill Data
www.thrill-data.com
The "lowest wait times" party having 35 percent lower average waits than the "highest wait times" party seems like pretty strong circumstantial evidence to me that the capacity isn't the same each night, although I'd love an expert like
@lentesta to weigh in if I'm missing something obvious here.
As for why? I don't think it needs to be a conspiracy. It's probably similar to the same thing they do/did with park reservations. They estimate how many tickets will sell, plan staffing appropriately, and that is the "sold out" metric. There's a reason why some parks are out of park reservations when it's not a crowd level 10/10 day.