Prosecutors often choose not to pursue additional charges against someone already serving a life sentence. This approach helps avoid wasting court time and resources on cases that are unlikely to change the individual’s circumstances or contribute meaningfully to justice (none of the murders for hire resulted in victims).

I actually wonder if those charges may still be on the table now that a pardon has been granted.

https://en.wikipedia.org/wiki/Prosecutorial_discretion

Being a drug kingpin is not considered "something relatively small" under US law, as you can see from the sentencing. Being the leader of a large drug operation and ordering hits to protect your business would be considered worse than trying to take out a hit for whatever "personal reasons".

Obviously the hits are a lot messier to prosecute as well with the misconduct of the FBI agents, maybe you could hammer that enough to confuse a jury. But people are commenting like the evidence outright didn't exist - I can only think they have either heard it told second-hand, or are employing motivated reasoning.