Of the two sets of charges, one was a much more easy conviction - the drug dealing.
It's not the prosecutor's job to introduce clouds of doubt into a prosecution. And this is not a case of "better ten guilty men go free than one innocent man be found guilty". He was, apparently quite overwhelmingly guilty of the charges related to running Silk Road. Why risk a conviction by charging him with something that might not stick (yes, I realize you can be found guilty of some charges and not others, but a whole world of doubt is introduced)?
