There were a number of reasons for this. Here are the two most important reasons that Wilson (and Pershing) had.
- The pride of the US Army. Putting Americans under the command of the French or the British would have been an insult to the American officers. It might reduce morale in the Army for this reason.
- The ability of the US to influence the peace. If the US forces were simply mixed in with the French or British, it might look as if the Americans had not really contributed much to winning the war. Wilson wanted the American contribution to be very clear. This would, he hoped, allow him to play a bigger part in shaping the peace treaty after the war ended.