What accounts for the fact that American involvement in Korea was more direct and intense than its role in the Chinese Civil War?
There are two major reasons for this.
First, China's civil war was a domestic conflict while the Korean War was an invasion. The North Korean attack on South Korea was a much clearer provocation than the communist ascendancy in a civil war.
Second and more importantly, the Chinese civil war would have been practically impossible to intervene in successfully. China is, of course, much bigger than Korea and huge numbers of troops would have been needed. In addition, trying to win that war would have been much less simple than trying to win the Korean War given the popularity of Mao and his communists and the fact that the Kuomintang did not have huge amounts of popular support.