Because he would not work well with their command structure. Being gay wouldn't help him, but starting out at the bottom of the totem pole means Johnny would probably kill someone out of sheer frustration, and then he'd probably end up dead.

Wiseguy by Nicholas Pileggi is a good one. So is Donnie Brasco by Joseph Pistone (the book, though the movie is well-researched too), and I like Mob Cops by Greg B. Smith.