One China policy, both Taiwan and China (ROC and PRC) was pushing for a one China under one rule, so when PRC flipped the seat from the ROC, this policy made the possibility of two China impossible. Also during the time when that happened, the Cold War was going on. PRC split from Soviet Union and recognized Mongolia’s independence while ROC did not recognize Mongolia’s independence, therefore the UN nations interests played a role in switching the official UN seat to PRC and not recognizing ROC. China was a strong communist state that just turned it back on the strongest communist state. As for now, Taiwan did relax its one China policy in order to get recognized as Taiwan, but currently the status quo is what the UN prefers because it avoid legal and politically consequences of changing the status quo. And it also avoids angering China and a possible military action. Right now I don’t think a lot of nations want to anger China.