It seems for many darknet dealers and operators it's often silly schoolboy errors that lead to their downfall. Ross Ulbricht would likely still be a free man had he covered his tracks a bit better when he was trying to set the site up.
It might also be that they feel like they are fully anonymous there, so they don't look at the details that have indeed lead to them being arrested. Nothing in the world is fully anonymous, so even there you have to make sure you don't leave any kind of sensitive information behind.
Did you even read the op? He got caught by trying to register the same username he used on the darknet so he probably would have been anonymous had he not willingly tied his real name to it when he stupidly tried to trademark the dealer name he used.
