Steve frowned deeply because he had apologized for that. He said it in the letter to Tony. That was still Tony's problem after all this time?
"I made a bad call about that. I said as much in the letter." He was protecting himself when he didn't tell Tony because he knew Tony would want to make someone pay. That someone happened to be Bucky, Steve's only connection to his past and his life long best friend. If there was one blind spot he had it was Bucky. "I don't know what more I can say."
He chose protecting his past over being honest. It seemed like an easy choice at the time. It was the wrong call but there wasn't anything he could do to fix it now. He didn't understand what Tony wanted from him now.