Question: why is nmos a good conductor of 0?
Answer: So node out reaching to a potential less than Vthp turns off the PMOS. So the maximum voltage level that the output node can be discharged to is |Vthp|. So an NMOS passes weak 1 and PMOS passes weak 0 whereas no such situations occur when an NMOS passes 0 and a PMOS passes 1.