They also have a high standard of evidence, before they can go to trial. Otherwise, they are better just observing, and waiting for more evidence to reveal itself. Once you shoot your mouth off, the suspect stops making mistakes.
The FBI is not good at finding [good] computer hackers. They have to outsource or bring somebody in who actually knows what the hell they're doing. And those people are not necessarily very good at it.
They admit in the document to "backhacking" the targets, which already puts them at an advantage against law enforcement.
edit: s/probably/probable (it's late)