Evidently, they weren’t unnoticed. They had been investigating him and decided to not press charges. Perhaps they held back to see if they could entrap a bigger fish at a later date?
Could be. With all the alternative theories floating about, one is that Manafort was a Clinton/Obama (FBI/CIA?) mole all along. I sort of doubt this at the moment, based on his indictment, but if he gets completely off the hook later for what does appear to be serious financial fraud, you’d have to wonder.