The time for action was before 1946 and 47 which saw the U.S ending the horrific war, and our military was stretched to the brink as we went in and occupied Germany, Japan, South Korea, and many other nations to keep the peace while governments were reestablished.

The U.S in post WW2 China actually used a lot resources to try and position the KMT for success in reoccupying Manchuria, and provided the army with a shit ton of aid, support, and even troops to help reestablish authority. But, despite this the KMT proved to be incompetent and often outmatched by the communists as the civil war picked up again in 1946. The U.S had been dealing with Jiang's pettiness, corruption, and lack of cooperation way back when General Stillwell was tasked with helping China.

While the civil war raged on until 1949, the complete inability for the KMT to put down the Communists coupled with the awareness that Jiang was leading an unpopular army consisting of former and present warlords made the U.S realize that the only way to "save"China would have been the deployment of millions of troops. Such a war would have been astronomical and probably would have casulities surpassing that of all of WW2 (See Japanese combat deaths in China), and there would have been no guarantee of success.

When the U.S did became deadset against the spread of Communism (late 40's), the KMT was already a dying force that struggled to contain the Communists, and by 1948 the final nail in the coffin was the fall of Manchuria in 1948. After which, the Communists quickly swallowed up the entire country in less than a year.

Because it didn't make any military sense to attack China, and the US was tired of war. They had just finished a brutal war in the Pacific against the Japanese Empire (And Germany/Italy, but that war was not like the Pacific War), and it was arguably one of the most brutal wars the US ever fought in its history. They had no desire to start another war, let alone another Pacific War, which the Chicoms would fight with brutality and efficiency. Besides, Mao was no fool. He had hid in the shadows while Chiangs forces fought and ground themselves down against the Japanese, and when they finally left, Mao began brutally assaulting the remaining forces. Chiangs men were outnumbered, outgunned, and just damned tired. They no longer had any realistic chance of winning, and the US recognized this. They weren't going to send their tired soldiers off to a losing war.

Mainly because US was having tough negotiations with USSR regarding the Post-War occupation of Japan.

If US intervened in China, it would have taken US troops from Japan, opening up Japan for USSR take over.

The public sentiment in China would have also shifted. There were a lot of Chinese, even in the ROC government, who were in favor of imposing heavy punishment on Japan after the War.

If US intervene in Chinese Civil War, the Chinese public would have sided with Mao and USSR.