The explanation is in the piece. Here's the logic: If you try to reduce demand for a product, but actively maintain a supply of that product, it sends a confusing signal to the market. If you want consumers to stop buying something, surely you also have to communicate that it will no longer be available. For as long as governments stockpile ivory, consumers are likely to buy ivory whenever they can, believing that the trade will one day resume. That undermines the credibility and therefore the efficacy of demand reduction campaigns. These campaigns try to convince consumers that their purchase of ivory directly destroys elephants. Why would they believe that line if they see African governments keeping ivory in the hope of being able to sell it one day?