Probably for much the same reason that “IF” you coat your saildrive with copper based antifouling (Dont ever do that, not even by mistake !!), the aluminium would corrode very rapidly in favour of the copper in your antifouling paint. The only time you would need a different (non zinc) anode, would be if your boat was sailing in fresh water. Take care to get your anode from a responsible supplier, as there are some cheap knock offs on the market which might let you down, and that would be a very expensive situation to have to rectify.
