It's because when people in highly-sensitive positions of power do things that they want to keep secret, they become a security risk by opening themselves up to the threat of blackmail.
Whether or not they should want to keep their whoring secret is a different matter between them and their families and their voters. I don't have a censorious take on that, I'm just telling you why it's a security risk and is newsworthy.
You say that as if you think it hasn't been tried before.