Its power was a victim of its own success. The high water mark for influence was probably the Kennedy administration, but at the same time it's a pointed moment of increased scrutiny from RFK's crusade. This would lead to things like RICO, which made it much easier to prosecute a much wider net. Former Outfit outposts like Vegas developed sufficient underworlds of their own, and pretty much just served to draw negative attention back home. National prominence isn't the best thing to have, if you're looking to operate a criminal enterprise. So it got lean.

But it was also a cultural thing. Now that there was money to spend, the next generation was directed towards legitimate careers, and that money started going towards legal purposes. There's plenty of business that's always been legitimate in Chicago that's built on mob money, even if only through some peripheral connection. Other migrants and immigrants started their inroads into street-level operations. And there was a certain degree of die-off of the old school leadership without anyone wanting to stay as big.

The really interesting aspect of that is in the ways that, cue the opening scene of The Godfather, the Outfit was an organization that filled certain social services in a community. Not only did you see that community in particular growing out of the need, but there are ways in which government and corporations really did start picking up the slack, incidentally denying the criminal powerbase - after all, when you can get a credit card, you don't need a loan shark.

It's still very much around, there's just not enough value in doing things that attract much national scrutiny when local control is big enough.