Due to being on the losing side of the Second Sino-Japanese War (WW2), the government of the Republic of China (China's government at the time) had to displace their government to new capitals several times, finally evacuating to modern Taiwan to avoid capture/defeat during the war. By the surrender of the Japanese at the end of WW2, the communist revolution in mainland China had taken hold; aided by the notion that the ROC government was extremely weak in relativistic terms at this point in history even prior to the war. The wake of Japanese destruction/occupation, the concurrent spread of communism internationally in the first half of the 20th century, failing Chinese trust in their own government and it's ability to protect them, and Soviet influence and assistance in arming the communists all contributed to the successful revolution and formation of the CCP as the ruling government of mainland China. The modern government of Taiwan is essentially the remnant of the old Chinese government, hence the CCPs interest in "unifying" it (not unlike Russia's designs on past-USSR controlled territories). Like all things in history, there were a number of factors at work and Japan wasn't solely responsible for the formation of the CCP, but it's not historically inaccurate to say that the ROC was already weak and the war with Japan broke it and fuelled a lot of the native Chinese dissent with the ROC government and subsequent embracement of communism. In a nutshell the way I understand it, I'm not an expert.
