It's the Chicago political machine at work. Everybody's corrupt at some level; you usually have to be to get the job in the first place.
Four of the past eight Illinois governors have gone to jail for corruption. Back when Blago was still in office, I looked up the list of his biggest campaign contributors and compared it to the list of largest state contracts awarded. They were exactly the same list, even in the same order. Apparently a campaign donation of about 20% of the value of the state contract you wanted was appropriate.
There are some honest and decent people that get elected to government, even in Illinois, and I know this because I have known some of them before they got elected. They might be naive academic liberals, but they're decent guys. However, the political machine is much, much larger than any one man. No one can stop it alone.
And I think that's why the US is so corrupt at a Federal level, despite who is president. We had a clueless conservative that we replaced with a naive liberal, and still nothing changed. The system's too big and the power is too well-entrenched.