It's locked to control server population. When too many people migrate to the same server they must give incentives to others servers to make them more appealing. Unfortunately, that appeal is "hey look! No wait!"

I don't necessarily agree with that mind set, but I see why they do it. I picked my server purely because it was open at the time of character creation. Just have to wait for other server populations to go up. I think if everyone would create characters on other servers, we could trick them into reopening the one we want.