It was a coward's move to not admit to her failings when she was confronted. Out of fear that Antonin would never forgive her and a legitimate concern that he might physically hurt her, or worse, Hermione didn't want to tell him the truth. He wouldn't understand why she never told him. When he repeated his question for a second time and then a third with anger in his tone, she knew she couldn't keep refusing to answer. When her husband wanted to know something, he was tenacious.