the study used only federal convictions to measure corruption. If you used state convictions too, I suspect states like Texas, NY and NJ would be higher on the list.
Southern states get more attention and oversight from the federal level ever since the civil rights era, not surprising they find more as they have more practice snooping around there. Also, when more of your state budget is spent on stuff that is ripe for graft (highways, construction, police, low loan guarantees for developers and investors) you will get more corruption than if you spend more on education, environmental, health and welfare initiatives.
and in the case of Hawaii - it helps if you had a very powerful senator in the form of Uncle Dan who for years who controlled the purse strings of the FBI and other federal investigative agencies.
but, yes, Southern States have along history of this stuff, going back to the variety of Englishmen who founded them (they weren't the puritan types), the gamblers, freebooters, fortune hunters and carpetbaggers that attracted, the greater gulf between the franchised and the not (more room for corruption), missing out on most of the industrial revolution (which inherently caused many states to create more oversights and audits, to improve efficiencies), greater traditions of political patronage (as corrupting as the ward politics in Illinois, both are non-existent in those least corrupt states), more big, widespread and flamboyant corruption (not little limited stuff like stealing postage stamps or quid-pro-quo requests for a single senatorial election) just to hit some highlights of historical differences.
Sometimes Southerners harbor a downright dislike of the lessor or petty and undistinguished corrupting behaviors of others, celebrate their own inefficiencies and local way "of doing things", sometimes to the point of being more forgiving of corruption. More folks there than elsewhere want to leave their mark on the world as a soldier, conservator of local church and customs, writer, musician, restauranteur, etc. than creating, improving good government and organizations, or sound institutional public practices.
Earl Long said his Louisiana constituents "don't want good government, they want good entertainment." ...When Edwards ran against David Duke for governor, I remember his supporters had bumper stickers that said "Vote for the Crook. It's Important" (those two both ended up behind bars)...In Tennessee two musician brothers once ran against each other for governor, and thier debates culminated in a one-on-one fiddling contest.
I grew up in the least corrupt state on the list, Oregon, and I'd much rather (not being a businessman, developer, or activist, etc) live in a more lively and interesting state like Louisiana or Mississippi or Alabama...
