Sentencing is complicated in the US. Generally speaking, they have a huge range and a standard for computing where one falls in that range, but everything within that range is open to judge's discretion. Life without parole was within that range for the crimes that Ulbricht was convicted of.

This standard is an enormous document, https://www.ussc.gov/guidelines which lays out the rules for adjustments. Evidence is admissible (by both sides!) for sentencing, with a lower standard of evidence and burden of proof, to either raise or lower the sentence within the very wide numbers of what the conviction was for. So the Judge in this case found that the lower burden of proof was met for additional violent crimes being committed (with Ulbricht's legal team having an opportunity to rebut), and that impacts the sentencing calculations.

Not a lawyer, but I have listened to US lawyers on podcasts.