Why were women granted the right to vote in 1919?

Women's suffrage groups had existed since the 1870s, but during the war it was hard to ignore their arguments. Women were serving in the war, taking over from the men in factories and offices, holding families together while the men were overseas, and working in voluntary organizations that supported the war effort. They couldn't be kept out of political life any longer.

Women got the federal vote in three stages: the Military Voters Act of 1917 allowed nurses and women in the armed services to vote; the Wartime Election Act extended the vote to women who had husbands, sons or fathers serving overseas; and all women over 21 were allowed to vote as of January 1, 1919. 

Hope this helped :P