So servers don't get overpopulated. They open and close servers to ensure that new players are evenly distributed. There hasn't been a lot of restrictions lately but as was said we're heading into official PS4 launch and three new worlds were added so it's not unusual that they'd temporarily lock the most populated servers to try and funnel some people into the brand new servers since transfer of existing characters to the new servers will not be allowed right away. Basically they're trying to avoid what happened at launch when the Legacy servers in particular pretty much burst at the seams and cause heavy server crashes and login issues. Mildly surprised they haven't offered the World Pass option that XI has to invite a friend onto your server.