The biggest issue is population. If you think a couple 96v96 fights are laggy, wait until there are 3,000 people on the continent, let alone 500 in one fight. Daybreak would have to implement serious server queues, and many people would have to wait to join Miller while everyone on Connery during US NA prime time is going "where is everyone?"

It's also because of the larger database of information that has to be kept for characters in an MMORPG than in an MMOFPS. Take WoW, for instance. Buttloads of inventory, quests, discoveries, professions, achievements, reputations, etc. Not that PlanetSide doesn't have a bunch of information as well, but the scope is smaller.

Even the vanilla WoW server Nostalrius eventually had populations as large as 18,000 but they had to write and re-write custom network software and database procedures to handle it all for a game that was meant to have no more than 2,500 on it. And it was laggy as shit a lot of the time as well. The draw distance scaled up and down based on population. At high pop times, you wouldn't see an enemy until you'd passed each other on the road.