The reason is probably because the average American didn't go hungry because of the loss. Also, Americans are, for all their diversity, fairly mixed and culturally similar.
OTOH, if you commit a behemoth like Austria-Hungary where 40 per cent of the country belongs to disadvantaged minority ethnicities and then you lose the war and the population is reduced to digging roots from the ground and making soups out of leather boots (I am not exaggerating), there isn't any reason to continue the existence of such a structure left. Only fear that the uprising may get you killed, but if the soldiers are demoralized too, they won't shoot.
OTOH, if you commit a behemoth like Austria-Hungary where 40 per cent of the country belongs to disadvantaged minority ethnicities and then you lose the war and the population is reduced to digging roots from the ground and making soups out of leather boots (I am not exaggerating), there isn't any reason to continue the existence of such a structure left. Only fear that the uprising may get you killed, but if the soldiers are demoralized too, they won't shoot.