They achieved strategic and tactical surprise, with help from the sycophants who Dougie had and his unwillingness to accept unfavorable intelligence information that did not conform to preconceived notions. Of course, having his two columns unable to support each other, and with the winter weather handicapping UN air support...
The Chinese had overwhelming numbers, and they were willing to absorb huge numbers of casualties (KIA and died of wounds DOW) to achieve their ends. They were able to operate on a shoestring, of course they lacked heavy weapons, significant armor, and medical services were minimal. Once the prewar lines were re-established the US was no longer willing to make a serious effort to go further north, and trhe Chinese were not able to dislodge them and the stalemate of the truce talks period set in.