well,to answer your question in one sentence: Because PVA fought surprisingly well...NOBODY including Mao himself saw this coming. the initial Chinese victories was so great that within months the PVA forces pushed back the UN force back to 38th. Those costly victories gave PRC decent amount of bargaining chips,as it would turn out, PVA would heavily depends on soviet supplied hardware in later stages of the Korean war,but the commanding structure would still remain in chinese hands.