In the 1940s, popular opinion in the US considered the Nationalist Chinese government to be corrupt and cowardly. There was considerable truth in this - Chiang himself was not particularly corrupt, but completely failed to rein in endemic vice and graft throughout his regime. As a result, his troops were underpaid, badly led, and unmotivated. The personal contempt and enmity of General "Vinegar" Joe Stillwell towards Chiang was also transmitted back to the US and into popular culture, exaggerating this perception.

The Communists, on the other hand, were seen as being uncorrupt and idealistic. The Communists carefully stage-managed what visitors from the US could see (in e.g. the Dixie Mission to Yan'an) and hid their own flaws from view.

As a result, there was no enthusiasm for US intervention in China. The war had been going for the last 20 years and showed no signs of stopping anytime soon; the US public preferred to forget about the entire thing (while still selling huge quantities of modern weapons to Chiang, of course).

The fall of the Nationalist government was a huge shock to the American public. The Republican party saw it as a useful tool and used it to batter the Democrats, accusing them of avoidably 'losing China', perhaps as the result of hidden Communist sympathies. As a result, Truman felt compelled to intervene in Korea when the North invaded. It was because of the rise of Communist China, and of the internal US domestic political squabbles that followed, that the US fought the Korean War.

No one expected the Nationalists to lose, the Nationalists controlled most of the country and represented China when WWII ended and the former Japanese controlled territories had to be occupied by the Chinese. Plus they had received a ridiculous amount of money and weapons from America while the Communists didn't get anything close to the same amount from the Soviets. Chiang should have won but there were a variety of factors that contributed to his loss, the most obvious was corruption, another one was for some reason he downsized his army after the Japanese were defeated since he assumed China was his, and another would be that he had lost many of his elite units and officers in the battles against the Japanese. On the other hand Mao never considered victory over the Japanese all that vital since he knew that the Japanese couldn't subjugate China and especially not once they opened up anther front against the Americans. So he bided his time and prepared for the war against the Nationalists the entire time. Chiang controlled most of the major cities but Mao had the hearts of the countryside people once the Nationalist administration mismanaged the civil government. Mao's army swelled to an enormous size after WWII as the peasants all flocked to him. Also you have to consider that in South Korea, a few American units were already present when the North invaded. The North obviously fired on these units so the obvious reaction from the U.S. government was to reinforce and support these entrapped units in the Busan perimeter. If those American units hadn't been present in South Korea, the North would have likely overran the peninsula and America would have been unable to intervene.