There is a good chance that they were traced when they tried to convert their BTC to cash. And this is the stage where more than 90% of the drug traffickers are caught.
That's very likely it. Even if they haven't been caught converting to fiat directly, the entities who end up receiving these tainted coins might try to convert their coins without realizing what they have been used for, and that's how law enforcement is a step closer to the actual criminals. Bitcoin is just a horrible tool to use for practices like these.
I actually believe that law enforcement is very happy with Bitcoin being so transparent. It allows them to solve cases that with a high degree of certainty wouldn't be solved if the criminals used cash. But then again, this isn't going to work for ever with how people slowly but surely figure out that there is no such a thing as anonymity when it comes to Bitcoin.