Populations in the US were more rural and there was a larger road network that was likely far worse than most city streets in Europe at the time... I think that might have had something to do with it as well... I know I'd take the balloon tires on the back roads around here that are gravel and full of ruts and potholes. Older French porteurs had pretty wide tires as well, so in another sense we're kind of talking apples and oranges here... any American racing bikes from the time would look more like the clubman I reckon.
Exhibit A: Jean Jean 1940 Porteur:
source:
http://velo-porteur-ancien.over-blog.com/
As for American racers of the time there are a few examples worth checking out.
Certainly the popularity of different kinds of cycling changed but it wasn't like the dark ages over on this side of the pond either.