Do you think if he had testified, he would have also been indicted for Jan 6th crimes? Maybe it was strategic. Better to get a possible 2 year max prison sentence than what he could have gotten with RICO charges. I have no idea how involved he was with the insurrection though, it's just a thought I had.
He could have availed himself of his 5th Amendment right. Would have made him look guilty in the public's eyes, but he would have been no worse off legally. And now that he's convicted of contempt, Congress could subpoena him again. Rinse and repeat.