Years ago (late 90's through early 2000's), my work team rallied together to get folks registered for an annual IBM business conference held at the WDW Dolphin/Swan/Y&B/BW Conference Centers. At the conference peak (2000, 2001), the event sold out at 20,000 registrations ... in minutes. There were many screens to complete and verify: IBM Partner number, attendee's individual IBM credentials, conference pass selection, payment processing and hotel room booking. Lots of moving pieces.
While we felt there was always stiff competition to snag a spot, we didn't encounter the number of technology errors still plaguing Disney's systems. I doubt this is really a technology problem (IBM had it licked 20+ years ago) ... but that Disney simply doesn't care. At least, they don't care sufficiently to make their website a priority.
Put another way ... why should they bother? The event is sold out. That metric might be the only thing being evaluated ... and thus, the present system is successful.