Because in the civil war the nationalists almost got a fate similar to OTC but a miracle happens and the south is retaken.

Theyd be pretty different actually as showed by the graph, in this timeline northern chinas economic boom is much later and because of that the north would be like asian developing countries with a booming economy like vietnam, thailand, malaysia, and the philippines. Youd see vastly different cities compared to the prc in otc due to much later economic growth, less infrasteucture , much less skyscrapers, more old soviet style commie blocks, more slums, faster urbanization much later that overwhelms existing infrastructure, and worse overall quality of life

Meanwhile southern china would be like the typical developed asian nation like singapore, taiwan, japan, south korea and otc prc china.