Because this happened while he was away in Iraq. His wife made the mistake, and he is taking responsiblility for it. Just as he said in the resignation letter, he discovered it when he was filling out the papers to support his confirmation.
Can you provide a source? The NYT says:
A former New York City official who knows the circumstances of the withdrawal said that the housekeeper, who had worked for the Kerik family for about a year, left for her home country two weeks ago.
Fox says Keric left Iraq 9-4-03. In any case, he was present for a year of her employment without checking or withholding.
I find it interesting that her departure is about a week before his nomination.
"His wife made the mistake, and he is taking responsiblility for it. "
He should have taken responsibility for his kids by knowing who he was hiring to take care of his kids. Blaming the wife won't work. If this guy doesn't know what goes on in his own home, I don't need him "looking out" for mine. He should be prosecuted as should anyone else breaking the law.