I'm going to guess that the server people working for SE have a good idea what the problem is and how to fix it. Yoshida keeps mentioning the duty finder putting stress on the servers, and he thinks that increasing the amount of servers will lower the impact on each server from the duty finder. Maybe he's right...

That being said, each server can only handle up to 5000 simultaneous connections at the moment, and the more populated ones like Leviathan and Gilgamesh are going to still have problems unless they actually allow more people to log on at once. They are concerned about stability, but that is hopefully something the extra servers can help with.