Rather than balance or any other similar reasons, I'm sure they're just too lazy to change it. Selectively filtering out some stats but not others is probably more difficult than just blanking them (as they do for relics/mythics currently).
Imagine the extreme case where they remove all restrictions and allow you to dual wield RMEs with full bonuses (including WS access for both and aftermath on the appropriate weapon). RMEs would generally become offhand weapons and in extreme zerg cases you might use two of them, but the resulting DPS boost would be relatively small.
PS. There have been dozens of threads about this made since the forums were opened, and I have posted in many of them in favor of the idea (still am in favor). However, given that 1H weapon users are in a better DD position than ever before, I think the timing is wrong to push this issue now.