SE seems very reluctant to add more servers while there is still underused servers in any of the geographical datacenters (NA, EU, JP).
They have added new servers to the EU datacenter twice now, but each time it happened ALL of the EU servers had a fairly high population.
While several of the NA servers are overpopulated right now, there are a couple of them with a fairly low population. I suspect SE is waiting for those to fill up before adding more NA servers.
They very clearly want us players to do our part in spreading out somewhat evenly among the available servers rather than all bunching up on the most popular ones.