Why is it okay for a married man to become a deacon, but not okay for a deacon to get married?

> Can. 1042 The following are simply impeded from receiving orders: > > 1/ a man who has a wife, unless he is legitimately destined to the permanent diaconate; > Can. 1087 Those in sacred orders invalidly attempt marriage. Why does only the first of these have an exception for the permanent diaconate? In other words, why is being a married deacon okay if you get married first, but not if you become a deacon first?