I like watching animated maps too.
Anyway KMT lost primarily because they lost the popular support by letting warlords extort peasantry, which was already heavily beleaguered by Sino Japanese war. And they were super corrupt. All the western aid that was supposed to be used against commies ended up in corrupt leaders' pockets. Communists did not squander the Soviet aid like nationalists did. Their forces were also less exhausted by the Japanese compared to nationalists. They also inherited the Manchurian industries and weapon cachets left by the Japanese. Combined with war torn Chinese economy hyperinflating and unpopular KMT conscripts defecting to CCP side, they won.