The majority of wvw content is provided by the players themselves, and when a team lacks players in a certain timezone, the map dies (becomes stale) by about 33% per team.
In the early days, we had huge servers & teams before a server would reach the full status, and lots of servers had a zerg in every timezone, around the clock. Fun good old times. I suspect the servers allowable population was drastically reduced at some point, prolly to address all the angry kids complaining about map queues during primetime, at the cost of killing entire non-prime timezones.