Opening a new server is far more complicated a decision than the ability/resources to do it. Creating a new server will inevitably effect the games community and cause more splits amongst the playerbase, and with the introduction of the blue server so recently and it still not having a proper PVP scene you may risk splitting the community even more and causing two servers to have mediocre end game scenes for a very long time.
If that happens we will be in a situation where Red is the only server that's had time to establish a community, which means new players all want to go on red. That leaves these two new servers under utilised and defeats the point of having them. The Devs know when to add new servers, it's based on community just as much as resources.