Literal decades of doing events have shown us that the 2359z start time is the best "catch-all" time that is neither too late for east coasters nor too early for west coasters. It's never going to be perfect. Let's say we start a ZLA event at 7pm pacific time. That's 10pm in New York, and we'll be less likely to see east coast pilots fly as they are now limited to shorter haul routes if they have a reasonable bedtime for themselves. Likewise, it's hard to ask an east coast controller to stay awake until 1 or 2 in the morning to accommodate. The 5pm/8pm time captures the most compromise to get people to show up.
Lowerkey events we'll start at more leisurely times, but the traffic levels are far lower to begin with.
Part of the FNO cram is that it's that it's a recurring event. That consistency during prime time really drives those numbers.