It's a good question. I'm not sure the US has been that much more successful than many of the places, when you take everything into account. Meaning: if you are literally an empire, then your governance structures very directly have responsibilities for executing in the areas which you oversee, which is a really high lift.

If you're a quasi-formal empire, on the other hand, like we have been, the bar for success is lower. Influence is a lot easier than literal control: if one of your vassals turns over in governance, you just pick up with the new sheriff, nothing much changes. A literal colony doesn't have that luxury: you lose India, or the Congo, and it's not your colony any more.

I also think there's much to be said about the increased reach technology has given centralization. It's possible to oversee vast swaths of the world in a manner it didn't used to be, so the center holds for longer, at least if the hold is somewhat looser than total domination and control.