Why did Wilsonianism fail at the Paris Peace Conference in 1919?
Woodrow Wilson was idealistic from the beginning of World War I; in fact he had proposed his famous Fourteen Points before the U.S. entered the war. Among the points was his call for the creation of a League of Nations which hopefully would solve diplomatic issues and possibly even eliminate the need for a war. This was one of several reasons that Wilson referred to the war as the "war to end all wars." Wilson, however failed to consider the practicalities of European diplomacy. France had previously been invaded (and humiliated) by Germany during the Franco-Prussian War; in fact the German chancellor Otto von Bismarck had arranged for the coronation of the first German chancellor at Versailles, thereby adding insult to injury. Although revenge was certainly a motive as the above answer implies, it was not the primary consideration. Rather, France hoped to cripple Germany and prevent yet another invasion. It was this consideration that devolved into the famous War Guilt Clause and reparations clause. There was no "reward" intended, only costs of the war, although they were calculated down to the pension of the last French soldier. The French representative to the Peace Conference, Georges Clemenceau, took a dim view of Wilson's idealism. He once commented:
God gave us the Ten Commandments, and we broke them. Wilson gave us the Fourteen Points. We shall see.
Wilson originally objected to Clemenceau's demands; but soon learned that unless he acceded, Clemenceau would not agree to his call for a League of Nations. He was so intent on creating his beloved League that he agreed to all of Clemenceau's demands.
As far as Japan, as noted in the above post, Japan had entered the war anticipating that it would gain German possessions in Asia. Japan left the conference empty handed, as did Ho Chi Minh, representative of the Vietnamese People whom Wilson had unceremoniously thrown out. The failure of the conference to deal with Japanese demands was a factor in Japanese Imperialism which led to World War II. Similarly, by acceding to Clemenceau's demands that Germany be crippled, Wilson inadvertantly gave a platform to Adolf Hitler who attacked the treaty as unfair to Germany. His attacks were a major factor in his accession to power. Similarly, his high handed treatment of Ho Chi Minh may well have been a factor in the advent of the War in Vietnam fifty years later.
Basically, Wilsonianism failed because it was too idealistic. None of the other countries at the Paris Peace Conference were interested in trying to have a perfect world that was based on everyone being nice to one another. The other countries wanted to either A) get rewarded for fighting in WWI, B) get revenge on their enemies, or C) both.
For example, both France and England wanted to make Germany pay for (in their minds) starting the war. Italy wanted to be given an empire. So did Japan. Only Wilson wanted an ideal world order based on countries foregoing their selfish interests and acting in ways that would be best for the world as a whole.