Because there are low-tech ways of dealing with the cold: thicker clothing, resistive electric heaters, fire. With heatwaves past a certain point, you need high-tech (read: expensive) air conditioners, which is out of reach of tons of people.

Historically, the only reason more people died from the cold was because it was really cold in places where a lot of humans live more often than it was really hot. With the Climate Crisis, that is about to flip around.