There are companies dedicated to tracing and deanonymising cryptocurrency wallets—most famously Chainalysis.com. These companies track all the coins that went through one of these mixers and mark them as suspicious, which makes it difficult to cash them out.

The reputable Tor marketplaces no longer take bitcoin for this reason. Usually they take Monero or other coins that have a mixing mechanism built-in for every single transaction, along with several other layers of privacy.