Market is full of interesting altcoins, which are more interesting in dark use because of their safety. I don't know, who will use bitcoin for doing illegal stuff, it seems stupid
Some people still use it because of price stability. For example Monero or DeepOnion might be more stealth and anonymous, but if you are a seller and sell larger amounts, you need to exchange those altcoins to fiat so you are able to buy material or whatever. And altcoins usually fluctuate even more than bitcoin so there is more risk for you as seller. Also if there is not enough volume and buy orders for your altcoin, you might crash the price by yourself if you have larger amount of coins to sell.