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.
Its not about the ability to pay for it, its about the fact that the resources to constantly power air conditioning year around are finite, and will eventually run out.