It was previously on freebuild, available to ranked players.
With the EULA changes, it meant it had to be accessible to all players.
And as we all know, world edit can cause massive amounts of lag, even for just one person using it, never mind the potential hundreds/thousands that could be doing commands at the same time.
I've heard of AsyncWorldEdit and heard it works really well, but I have no experience nor have I read any facts on how well it would work on such a large server.
World edit has been requested many times, and I'm sure it's one of the reasons why many people don't bother playing on freebuild so it's definitely something that should be looked at.