A couple of things I found interesting -

Most if not all of the domains expire at different times. The majority of them are "parked" meaning there's no active website. They use a company called MarkMonitor (which is kinna funny) who basically monitor all of their domains and prevent people from registering them when they expire. They would step in if someone tried to register a domain they foresee registering in the future and squat on it in an attempt to sell it to WWE down the line.

Someone mentioned wrestlemania51.com not being registered. I can almost guarantee you that if you registered that domain you'd get a letter in the mail from MarkMonitor almost immediately.