Why did Vashti refuse the summons of King Xerxes?

According to the Book of Esther, Xerxes was celebrating the third year of his reign with a long party. On the seventh day of the party, Xerxes was drunk and called for Queen Vashti to be brought for the people to see, because she was fair to look at. Esther 1:12 says that Vashti refused to come, and Xerxes was angry.

The Book of Esther says no more, so we can only guess that the reason for her refusal could have been his drunken state. We can not look outside the Book of Esther for an answer, because historians say that there seems never to have been a Queen Vashti, and that Amestris was queen until well after Xerxes' third year as king.