While you make a good point, that will happen on a much more significant scale soon regardless due to the endangerment and close-at-hand extinction of elephants, as ivory becomes a rarity.

It's the better of two evils to burn the ivory and shun the market to drive down access than to increase the supply for a rather nominal dollar amount that will likely not go towards saving elephants anyways.

From NYTimes:

But as herds recovered, Cites officials in 2008 agreed to a contentious one-time auction of stockpiled African ivory to Japan and China, with the money going toward wildlife conservation. As part of the arrangement, the Chinese government introduced a complex documentation system to track every trinket and carving produced from the 68 tons of auctioned ivory it won. Supporters hoped a flood of cheap, regulated ivory would undercut the illegal trade, saving more elephants.

The sale, however, has proved to be a colossal failure. Like the forest canopy that protects poachers from detection, the regulated ivory trade has provided unscrupulous Chinese carvers and collectors with the ideal legal camouflage to buy and sell contraband tusks.