Why doesn't it snow even when it is below freezing?
Farrenthorpe is right, a little surfing the net for snow distribution would clarify the picture for you. However, I think you are referring to certain times when it rains further north and snows further south. There are several reasons for local anomalies, of which the most important is the dynamics of the polar jet streams. These are strong high-altitude wind that blow constantly in both hemispheres. These winds have meanders which migrate around the world, as depicted in:


https://www.google.com/search?q=jet+stream++image+OR+picture+OR+diagram&amp;lr=&amp;as_qdr=all&amp;tbm=isch&amp;tbo=u&amp;source=univ&amp;sa=X&amp;ved=0ahUKEwiX44v3tbHJAhUEjywKHSncCP4Q7AkINA&amp;biw=1352&amp;bih=631#imgrc=LXFiBx8qaJg5xM%3A

Meanders in these polar jets pretty much control the mid- to high latitude weather within the troposphere, bringing bulges of Arctic air south, and counter bulges of warm mid-latitude air to further north. But then they move on, and the reverse conditions sometimes prevail. As a long-term average the latitudinal control on snow distribution dominates. Of course, there are other influences as well, mainly related to heat re-distribution by the great ocean currents.