The same reason exists to expand Legacy gear as there exists to have any more regular gear than one set: Choice. Maybe people want to have a more extensive choice of appearance for their legacy sets. There doesn't need to be any more of a reason than that.
Maybe some people want to have some armor sets specific to type. Maybe some people don't want sorcerers running around in "heavy" armor. There doesn't need to be any more of a reason than that to leave some armors non-adaptive.