Given the history of European and Russian intervention in Chinese affairs ( e.g. the Boxer Rebellion, Sino-Russian border disputes and foreign mercenaries involved in the suppression of the Taiping Uprising) it does seem surprising that there was not more international involvement in the Chinese Civil War.
There was even a precedent for intervention against a nascent Communist state in the Russian Civil War where the 'White' Russians had received heavy support from European powers and Japan.
However, we shouldn't completely ignore foreign influences. The Chinese Communist Party was heavily inspired and funded by the Soviet Union while the Americans favoured Chiang Kai-shek. The policies of both the Allies and the Japanese occupiers from 1931 onward clearly impacted on the relative strength of the Guomindang and Communists and both sides had to consider the pros and cons of antagonising the European 'settlements' that had been established in Chinese cities.
Nevertheless it is clear that foreign intervention was less significant than in previous internal Chinese conflicts and an explanation is needed for this. Three major factors in foreign inaction are as follows:
1) The Japanese invasion of Manchuria - Tensions between the communists and Guomindang were at boiling point in the late 1920s when the breakdown of relations at the end of the First United front saw a widescale purge of Communists. This period of conflict would prevail through the 1930s (as evidenced by the Communist 'Long March' North to avoid extinction). Only with their common enmity towards the invading Japanese would the Communists and Guomindang temporarily suspend hostilities. This delayed the final conflict in the Civil War until after the defeat of Japan by the Allies.
2) League of Nations inaction against Japan - The League of Nations was discredited by its inaction in China. It condemned Japanese aggression but had no response when Japan withdrew from the League. This ensured that when the Chinese Civil War reached its final stages there would be few international or foreign troops on hand to intervene.
3) War Weariness after WW2 - World War 2 was the most destructive conflict in history. Following the war the major European powers all had to priorities rebuilding their economies and societies. Simultaneously many Imperial nations opted to attempt to regain formal foreign colonies that had broken away during the War (E.g. Dutch Indonesia and British India). The USA was left in a stronger position but also had significant global interests such as the occupation of Germany and Japan and providing economic assistance for Western European reconstruction. The cumulative effect was that few of the major powers had the funds or energy to intervene in China's conflict.
Given the above factors, why the intervention in Korea?
The Korean War began only five years after the end of WW2 but in the meantime the political climate had changed considerably. Tensions between the USSR and the other victorious Allies had increased greatly as it became clear that the USSR would impose communism on its military conquests while America and Britain would resume their traditional opposition to communist movements in Western Europe.
Meanwhile, the victory of the Communists in China focused Western attention on the area. Amid the increasing tensions of the Cold War and rising concern with what what would later be known as 'domino theory' the USA feared the consequences for other Asian nations if China secured victory for the communist North Koreans.
In summary, the shock of Japanese invasion delayed the final reckoning in the Chinese Civil War until the Allies and USSR were war weary and distracted. By the time of the Korean War the new pressures of the emerging Cold War made intervention seem far more necessary to the USA than it had appeared in China only a few years earlier.