Dude, it's very simple, it's called POLITICS. Are you this naive to think or even believe that by 1949 the U.S. wouldn't have been able to essentially stopped ROC from gaining the permanent UN seat if it wanted to since the formulation of the UN and it's Founding charter were created by the FDR and his wife, Eleanor Roosevelt. By the end of WWII the U.S. was the undisputed KING in military and economic power, half of the world's GDP was produced by the U.S. which is also why the UN is situated in NYC, USA.
When the U.S. established the relationship with the PRC in 1972 the American domestic politics didn't stop hating on Red China. And the eventual ejection of the ROC from the UN was calibrated with U.S. domestic politics in mind, as an American citizen you ought to have been more cognizant of this fact. Politics exist in everything America does. So what happened at the U.N. general Assembly can be described as a dog and pony show. So the idea that the ascension of the PRC into the UN was mainly due to the inner workings of rules and bylaws of the UN is hilarious and naive.
