The best explanation is found if you ever saw the movie “Goodfellas” The scene when Henry Hill and Jimmy Conway were going to throw the guy into the lion cage in Tampa. The guys sister worked for the FBI, so Henry and Jimmy ended up doing time.
In this case, there's the emails and according to this that's all they based opening a case on. That's a far cry from someone who admits they went to a loan shark and can prove they've been paying known felons with connections to the Mob.
I guess they're awash with money these days, though, so maybe it's not difficult to justify putting man hours into something like it has been sometimes in the past.
Regards