I think the original reason is as simple as the fact that so many people have already hit level 50. That and the fact that they'll have the lion's share worth of glamors available to them.
I personally agree that the level restriction should be loosened, and it should allow anyone of level 15 or higher to glamor their gear. The arguments people have listed against it are unbelievably silly, and probably more an attempt to play devils advocate than any real attempt at making a good point. After all, once you've got at least one level 50 character, there's nothing to stop you using glamor on your level 15 newbie gear for your next job, is there?
It hurts no one, has no impact on gameplay other than cosmetics and there are really no good arguments as to why it should remain a level 50 privilege.
