Because that is the linchpin for the conservatives. The drug war is the one thing that makes them complicit in the liberal's abuse of the commerce clause. Get that out of the way, and we can attack the root cause of the problem. Going after individual agencies is simply attacking the symptoms and ignoring the problem. If we absolutely have to have a federal drug war, then let's do it right, and pass an amendment.
ALL drugs have a risk (some much greater than others), society has made alcohol an acceptable drug. If you outlaw one you must outlaw them all. But if you remember we already tried alcohol back in the 30's.