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Perhaps because Wilson was an ideologically rigid, umsympathetic and impractical man, and may even have been already mentally damaged by a small stroke or two. He did not make a good personal connection to Clemenceau, and probably did not convince the Tiger that he (Wilson) understood France's special concerns with respect to Germany. That, along with France's traditional rivalry with England, may have convinced Clemenceau that France would have to push for hamstringing Germany economically in the present, rather than attempt to rely on guarantees by the Great Powers (including especially the United States) in guarding against a resurgent Germany. (And of course, in view of the events of 1940, he was entirely correct.)
Wilson did not seem to connect with Lloyd George either, and did not grasp the practical concerns of France and England, and seemed to have fetishized his League of Nations -- and "international law" generally -- to such a degree that led him to give up almost all of his other points to gain British and French support for the League and related formal apparatus, which, ironically enough, he couldn't sell afterward to the US Senate. It was perhaps as a result of this intransigence and lack of insight into the motives of others that caused Wilson to waver in his support for self-determination, which in turn led to a fair amount of hard feelings among those he let down.
In any event, Wilson's failures of leadership, at a time when only the United States could have exerted it, are probably one of the best reasons for the failure of the Paris Peace Conference, in the sense that the treaties and agreements and arrangements made there not only failed to preserve general European peace, but according to many points of view nearly guaranteed a second World War twenty years later.