Its much simpler to have one map for all arenas.
Zainos , please correct me if I am wrong.
Having multiple maps and 1v1 arenas on PVP is simple because PVP is hosted on one server. There are multiple sg servers, however. This means that a copy of each map must be stored on each server. If you were to have a different DM for each arena, Zainos would have to edit the files on each and every SG server, including the Solo SG servers, to make each arena have its own deathmatch. That includes setting the spawns on all of the arenas, and testing to make sure that the arenas work properly and that nobody gets teleported to the wrong arena. This would make so much more work every time an SG map was added since it too would need the DM added. The amount of time spent making, programming, and implementing could be better used in other areas of the server