Mao received oodles of support from the USSR, the Long March was the Reds fleeing northward to where they could still get that support, and then being able to punch further East.
The KMT was also a revolutionary force (against foreign domination), and initially preferred by Moscow for practical reasons - the USSR would at that time only support a joint command.
Mao became leader of the Reds because the Chinese Red's intelligence officer who was their contact with the USSR and held the codes for that defected to Mao from the previous leader (who wasn't going to admit to defeat by the KMT.) After that, the money etc could only to go to Mao, since he was the only point of contact the USSR then had with the Reds.
Post WWII Soviet support, esp military material support became very large indeed, allowing the Reds to crush the KMT.
But it wasn't helpful for either side to reveal the connections; better to portray the revolution as a mass movement of the peasants and workers than show all the gears and wheels.
Mao spent plenty of time and effort downplaying everything the USSR had done for him and in massive spats with the USSR, for his own political reasons. Just as Lenin didn't credit the massive support the Germans had given him, to undermine the Czar.