At the time, picking Garland as AG was a giant fuck you to republicans to get revenge on them denying Obama the supreme court nomination in 2016, a way of saying "ha, ha, you denied him a seat and now we gave him one that's almost as good."
Unfortunately, in hindsight it turns out that when you put a very moderate, nonpartisan, old-school Republican in the cabinet, they will run their department like a moderate, nonpartisan, old-school Republican, and that resulted in the DOJ focusing on the mooks more than the masterminds out of fear of being seen as a political hatchet man.