^I think the idea was that the League had been pursuing its plan to destroy Gotham for so long, and before that tearing down earlier cities in the same way, that a mere eight years of reform and stability didn't convince them that they could abandon their centuries-long program. They were too committed to their belief that it was necessary to destroy centers of decadence, and like all fanatics, they wouldn't change their minds just because the facts contradicted their assumptions. They surely saw Gotham's reforms as a temporary correction that would fail to stem the inevitable tide.
Indeed, one could argue that the corruption never went away. The stability was built on a lie about Harvey Dent and Batman. The Dent Act had been enforced so aggressively that it threatened to undermine civil rights. And the one percent were still getting richer at the expense of everyone else, as Selina pointed out. That's the key thing. You're defining corruption to mean graft and illegal acts committed by people in the government. The League defined it on more of a society-wide scale, feeling that America as a whole had become incurably corrupt. A global organization like the League wouldn't have been concerned about one city alone. They targeted Gotham because it was the biggest, wealthiest, most powerful city in the Nolanverse version of the US (President William Devane called it "our greatest city" or words to that effect) and thus the keystone whose removal would cause the rest of the civilization to collapse and allow the next civilization to arise.