You can use money to buy illegal stuff, you can use gold to buy illegal stuff, so it does not mean anything if someone uses bitcoin to buy illegal stuff. Bitcoin, fiat money, gold, are just different forms of the assets.
Indeed, the difference is only a way every perpetrator of a crime uses bitcoin, money fiat gold and others. You can use money fiat and gold as a transaction system for illegal goods if you meet directly with the seller, I mean you can only make transactions if you meet or know your identity. Unlike Bitcoin, you can keep your identity private from others and this is what causes everyone to get bitcoin, an easy-to-use tool for transacting illegal stuff.