Drugs dealers do work mostly with cash, but it has significant downsides - actually meeting your dealer/customers is much more dangerous and much more likely to get you caught than Silk Road ever was. Also, note that people are using less and less cash. At least where I live, this is significantly because of the untraceability of cash: shops reduce the risk of robberies by taking payments in electronic forms as much as possible, so they offer discounts and convenience - and customers respond.