Just like people think the Securities and Exchange Commission don't mess up sometimes
First of all, the SEC is underfunded, understaffed and, more often than not, has some sort of conflict of interest. The FBI is none of these. Secondly, if the SEC starts an investigation, they usually get their guy. The FBI did an investigation here and came up with nothing.
Look, if you want to assume that a former university athletics director and a former college basketball player/sporting goods rep/real estate investor somehow outsmarted or bribed or otherwise slipped through the grasp of the FBI, that's fine. I choose not to give them that much credit. The more likely story is that the feds cast a WIDE net with their investigation and rumors started flying.