Regarding China, with the amount of air pollution and the sheer volume of people who still smoke, overall, their lungs are not exactly in tip-top shape. This virus hits the lungs hard, so they are already facing an uphill battle from the start.
And regarding your observation about cases north of the equator being far greater, the seasons south of the equator are also opposite of ours, so the warmer weather is probably playing at least a part.