You're assuming the ROC actually controlled mainland China in its entirety. The only reason it took so long for them to be forced out was the Japanese starting the Pacific theatre of WWII. You can't really fight a civil war and expect to fight another empire at the same time.
The temporary halt to the power struggle over mainland China was just a self preservation tactic on the part of both parties.
The temporary halt to the power struggle over mainland China was just a self preservation tactic on the part of both parties.