Because it makes no difference.
See, Detore didn't testify before the jury at Traficant's trial. He may have been subpoenaed, but he said he was going to take the 5th. The jury can only vote for conviction or acquittal based on the information they are presented, and because Detore refused to testify at Traficant's trial, Detore didn't allow the jurors to hear his story. Once the case went to the jury, Traficant--and the jurors and prosecution--are stuck with that vote.
Of course, if Detore had testified before the jury at Traficant's trial, it would have exposed him to cross examination by the prosecution and possible testimony from others that may have rebutted his testimony. He didn't get that testifying before the Ethics' committee. He still exposed himself to considerable risk, but the risk was much less had he testified at Traficant's trial.
I don't think that is the way it went. I read in the Plain Dealer that the judge ruled Detore and other witness could not testify. Looking for the link now...