Answer: because she is from a lower social class.
We encounter the story of Gualtieri and Griselda in The Decameron. It is the tenth story in the collection. Gualtieri is a wealthy marquis who has to marry. He decides to marry a poor, but good girl from the village. Griselda is a fantastic wife, but at first she is not accepted by the subjects because she is from a lower class. Gualtieri tests her in many different ways, and she eventually proves to be the ideal woman of the time.