Balmung is the only server to have a distinct split in demographics, being RPers and non-RPers. There's no way to predict that a designated RP-server will reach a population that's high enough to necessitate server locking. You can make the argument that it might reach cap eventually, but that point is basically meaningless.

Balmung was created as a result of a server merger. The argument that an RP-designated server will quickly fill to capacity isn't really relevant because this time, there's no pre-existing PVEr populace to add to the server.

Balmung is the RP hub because Besaid was the RP hub before it, and Besaid got merged with Fabul. Roleplayers have been asking for a designated RP server since 1.0 because this sort of thing was bound to happen.