Even if the temperature is lower on Sunday, the ice bowl was windier AND the wiring to warm the field failed in 1967.
Think about it, millions of people who would normally be outdoors will be indoors watching the game. That means a lot less body heat to warm the outside. Thus the temperature will drop further. Back in 1967, we didn't have quite so many people watching on TV. Many people were out hunting, ice-fishing, chopping firewood and things like that.