Why does the Duke of Vienna give up power at the beginning of Measure for Measure?

At the beginning of Measure for Measure, the mild-mannered duke of Vienna lets everyone break the laws with impunity, particularly one harsh law condemning to death any man who lives with a woman not his wife. Men and women are living in sin now and not marrying. The duke wanting to change this, but fearing he will be unpopular, decides to charge Angelo, a strict and moral man, to enforce the laws and pretends to leave on a trip to Poland.