I don't know why, Redman. Our usual winters will hit 25 once or twice with maybe a couple of other 30 degree freezes. Some years it doesn't freeze at all. The last time we got a real polar vortex dipping this far south was 2014, when it got down to 15 degrees and never above freezing for three days. That was very unusual.
Maybe it is the concrete and autos over your way. I know that New Orleans, 60 miles west of here, is usually 10 degrees warmer during these temperature dips. OTOH, that may have a lot to do with Lake Pontchartrain.