It's not. All of the DNMs have switched to Monero already. Ransomware is also changing to Monero. Anyone using Bitcoin for illegal stuff at this point will be caught.
Correction: Anyone running a large business using bitcoin for illegal stuff that isn't based some place like Russia is likely to be caught at this point.
Even the 1Feex wallet that has the stolen Mt. Gox funds (now worth $5B) hasn't been touched. Bitcoin has been radioactive for illegal stuff for a while.
Or the owner might just have died or might be in prison. I'm pretty sure that the reason for not moving those coins is because BTC is radioactive. It might be partly a reason, but surely not the only reason. If you really needed the money you could try several methods to launder them.