Can someone explain how a BTC tumbler can fail to obfuscate the history of a certain BTC wallet? Seems like such an easy way to basically blame everyone involve

Can someone explain how a BTC tumbler can fail to obfuscate the history of a certain BTC wallet? Seems like such an easy way to basically blame everyone involved in the tumbler which is not how prosecution works. A defense counsel tactic similar to this is to put doubt in to _which_ of a handful of individuals did the crime when each is blaming the other, and that doubt is enough to be found not guilty.