I don't know for sure, but I'm guessing it's probably the wrapping around a weapon/limb issue, a safety thing. I have played in LARPs that allowed them, and the "chain" would frequently tightly wrap around whatever it hit, and made it easy to physically disarm someone (which that system allowed). The chain was made of fabric, open cell foam, and duct tape to create the look of a chain.
Alliance does not allow weapon trapping or intentional physical disarming, which is the point of the chain mace.
Edit: added more explanation.