why did american aid to jiang's nationalist fail to prevent mao's communists from taking control of china?

I take it you meant Chiang Kai Shek, the all forces General in China.

well, Chiang Kai Shek government was riddled with corruption, many or most politicians were on the take or grabbing whatever they could, and that included American aid, and they did so.

Add to that dissatisfaction by the populace of such abuses by the Chiang Kai Shek's regime, he wasn't exactly light-handed either.

so, in the end, his government became a turn off to folks, and the "promises" from Mao were very attractive, that said, Mao fought the Japanese as well as Chiang Kai Shek, and so that added to folks leaning towards Mao.