That's a very astute observation, PB! Pistone was obviously getting sucked into the Mob life big-time. That came through loud and clear in his writing about his relationship with Sonny Black. But even if he weren't getting sucked in, what good would have come of his being made? At minimum, he'd have been asked to be involved in more big-time felonies than he admitted to during his "unmade" career.
I got the feeling that his FBI superiors pulled him out precisely because they saw the danger of his becoming a real Mob guy.