The US didn't like the KMT that much because despite extensive US support they weren't all that great of a fighting force, due to heavy corruption.
From what I remember Chang Ching-Kuo tried to do some anti-corruption efforts but it was just too much.
Chiang Kai Shek was a warlord ruler of warlords, his subordinates sold their rifles, sold heroin, and were more focused on the communists (at first) than the Japanese.
On paper, the chinese army in 1945 was very formidable.
It had millions of men deployed (true), millions of combat veterans (true), and had fought a great power to a standstill (true).
It's army was also made up mostly of poorly paid conscripts who were given little training, and non standardized equipment due to the government trying to scrounge together whatever it could get.
The army was exhausted from 8 years of constant fighting, and the corruption that had been endemic to the KMT had metastasized in the form of Chiang declaring martial law and permanently seizing executive power.
He had no commitment to democracy (not unusual for a nominally democratic US ally), but was not capable of holding together his country through an iron will, such as stalin had.
And there weren't any alternatives to Chiang, as the left wing of the KMT had defected to japan and treated accordingly after the war (read:shot).
The reds had meanwhile managed to keep their forces in tact through the war, and cracked down on corruption and drug trafficking (by murdering drug dealers and burning opium dens with their customers still inside, and seizing the means of production.). They acquired literals tons of military equipment left by the japanese, and faced a demoralized KMT that had no chance of resisting.
And sure enough, the CCP cut through the KMT like a knife through butter.