Just like most habitual criminals, even serial murderers — focusing limited resources on the easier-to-convict and sufficient-jail-time crimes. Nothing says they have to prosecute every single crime. So, yes, just because it isn't prosecuted, doesn't mean there wasn't a crime committed, it just didn't result in a conviction.

They already got a life in prison and forfeiting $183,961,921 [0]. They don't need more, and there are other criminals to prosecute.

[0] https://www.justice.gov/usao-sdny/pr/ross-ulbricht-aka-dread...