TL;DR, a warming planet causes the jet stream to weaken. The jet stream keeps cold air in place and weakening allows it to move farther south.

Because climate isn’t exactly equal to weather. There are effects on weather from what happens to the climate, obviously, but a warming planet doesn’t only mean warm winters. It means instability.

There is a more-or-less constant stream of air moving across the US. This stream of air is pushing the cold air at the north pole up north.

As the earth warms, this stream of air weakens. It can’t push the cold air up north, which causes the titty that is the polar vortex to sag ever southwards.