Yea as far as I understand it, they have a lot of work to do, and that includes collecting evidence. In order to do this, they have to go through a lot of trials and attempted indictments of many different low-rung people to amass the evidence they need for the higher courts to take their main case seriously. I'm not saying Sussman is low-rung, but they certainly are not aiming for indictment alone. Durham has a grander plan.