UN was created in 1945 and at that time only one china was there which was called ROC i.e. current Taiwan. So there was no question of PRC as it was not there.
Now lets see what happened after 5 Year. After five year communist party (MAO) controlled most of the china, which was called PRC and original government i.e. ROC was moved to a Tiny Island called Taiwan which retained the Permanent Seat of Security Council.
But we should remember why China got the permanent seat in 1945, the reason was it was the most populous state in the world, and PRC was the true representation of that not ROC. Slowly China started getting close to US (thanks to the border dispute between China and USSR) and with the support of US and some Latin American countries, by 1970 UN recognized PRC as the true representation of China and awarded the UN permanent seat.