If you take a look at any time zone map, for example timeanddate.com/time/map/ and assume prime time is around 8 pm (That equals -9 to -14 hours from UTC).
Then you will see that 5-10 am UTC is actually primetime for not much more than the pacific ocean and the islands in it. Sure there's some parts with a higher population in these vague areas, but it's far less than what you're describing as "tons of people".
Maybe it's not considered "prime-time" for those five hours for many places, but for many people those five hours are the best slot they have to play and there's nothing to do. The population winds down after 5 UTC because there's no end game content, not because there isn't people online. For those who can only play during these five hours I feel really bad for, because they essentially don't get to experience the fun in the game when there's no end game content.