The Depression. It would have prevented any sort of armaments spending in the 1930s and by the time that FDR had any sort of political capital to increase spending on the military it was already 1937 and the navy was the service that congress was willing to favor. It took Hitler getting the Sudetenland before any army spending was passed:
Much of that had to be spent just getting modern equipment, as the Depression had left the US army with WW1 leftovers for the most part.
Effectively though if they were going to put together an expeditionary force it would end up using WW1 equipment and French stuff if going in 1939. Likely they wouldn't be able to ship until 1940 and then would have only about 200,000 out of the 385,000 in the regular army+national guard, which means if/when they are defeated and lost it would effectively destroy the nucleus of personnel needed for a major expansion of the ground forces. Left over units could work up over time, but we're talking about the least deployable elements, which means you probably don't see any US forces even able to be deployed abroad until 1943.
The Marines BTW were less than 20,000 men as of June 1939 IOTL, which included over 4,000 men in the fleet air arm. So literally a single division. No way that would be deployed given their expectation of having to guard against Japan.
Certainly an AEF would help the Allies in France, but they'd still be working up for front line action by May 1940 since they'd be arriving the same year and probably would just end up caught up in the retreat and have to compete with the 2nd BEF to escape. We know the Brits would favor their own forces over their allies and the general disaster of the situation by June would mean the US forces probably just end up getting used up the vain hope of stopping Case Red.
