other factor was the soviet aid, according to the archieves,Liu Shio Chi (who later died in purge during the cultural revolution) admit the soviet aid play important factor in Communist victory,for example more than 300 plus capture japanese tank .two factory producing helmet not to mention million of ton of ex-japanese small arm and artillery pieces capture by the soviet armies were illegally hand over the CCP.before that more than half of Mao's red armies were armed with nothing more than some sword and spear, since arms were hard to come by.
that's nothing compared to the amount of aid truman gave the kmt. in the end, the communists won because they had the support of the people and they had loyal troops, plain and simple. in the words of mencius: 得人心者得天下。
every member of my family who lived through that time, (even the ones that bear a bitter grudge against the ccp) can confirm that mao had the people's support. think about it, mao's support base was the peasants and the sharecroppers (80-90% if the population) and also the liberal intellectuals, while chiang's support base was the rich merchants and the elite families.