I think heroin has much higher dangers and usually the accidenal deaths are very swift and come from nowhere. Alcohol on the other hand kills a lot more people but because it does so slowly they have higher chance to pick themselves up, go to detox, AA, etc
This is an absolutetly 100% valid reason, if not the best reason, to make a substance illegal. 90% who try get addicted, so better keep people, especially young ones, far from it.