Why can't the law enforcement take down all the illegal dark web markets using the network investigative technique it used to shut down 'Playpen' in 2015?

​

This question might sound silly to many of you, but I am not related to the cybersecurity field and hence this question. If they can shut down Playpen using that technique, couldn't they use the same to take illegal markets down?

cybersecurity dark dark web dark web markets down law law enforcement markets network web