People use dark web for conducting illegal business and buying illegal stuff, this simply states that Bitcoin can be used for illegal activities.
The main reason bitcoin is falling out of favor on the dark web has to do with the sudden increase in the cost of transacting in bitcoin. Here, again, it helps to get into the technical details for a moment. All bitcoin transactions are kept in a decentralized and public ledger. When someone makes a transaction with bitcoin, miners in the network solve cryptographic puzzles to verify and log it—and get paid a small fee in bitcoin to do so. That has given the cryptocurrency a scaling problem: As demand for transactions has gone up, the price to transact has gone up. Indeed, the price of a bitcoin transaction recently spiked as high as $55.