The main reason that Wilson was forced to compromise on the 14 Points was the fact that European leaders were adamantly opposed to some parts of the 14 Points and Wilson had no way to force them to accept.
In particular, the British and the French wanted Germany to be punished for the war and to be kept weak for years to come. This was contrary to Wilson's ideals. However, these were essential to the British and French leaders both politically (because people in their countries demanded it) and personally (because they believed in it). Therefore, these leaders forced Wilson to back down from his idealistic proposals.