Saftey reasons. As with any building can only hold so many people, so their is a timed entry to keep count of the number of attendees at any given point (includes staff) in case of a fire, etc.
Safety
Safety. To spread out the crowds.
To prevent overcrowding which could be a fire hazard and to allow those individuals with tickets an opportunity to see everything they wish to see.
I believe it's for safety reasons
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