Why was Chiang Kai-shek defeated in the war against the communists?

Let us recall that Chiang Kai-shek was at the head of the Kuomintang party, which was a supporter of the pro-Western path of development. They fought with the Chinese Communist Party, but the forces were unequal, because the Soviet Union supported the Communists and even returned the captured Japanese weapons to them to fight the Kuomintang.In addition, partisan units controlled by the Communists were operating throughout China. Moreover, an increasing number of people joined the communists due to the fact that the Chinese Communist Party (CCP) was preparing agrarian reform and reduced the rent charged to the peasants. Most Chinese people now firmly believed that the Communist Party was capable of making significant changes in the country to improve the lives of the entire Chinese people.