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.
I lived in hot climates including florida, and when the electricity / AC shuts off for even a few hours, dozens of people die.
From my experience in a formerly temperate place (Vancouver, Canada), adding an AC in a house or in our family’s case, townhouse/apartment (it wasn’t needed just a few years ago and the majority of older residential buildings don’t have any), is a gigantic pain in the ass. You have to deal with the exhaust heat and strategically place the AC unit, which are almost always those inefficient portable units. Compared to if it’s cold and you don’t have sufficient heat, just get an electric baseboard heater, fan heater, infrared heater, electric blanket, whatever for one tenth the price and whjch could well be carried around in one hand while being powerful enough to heat an entire living room, and place it basically anywhere you want warmer than it is currently. Obviously if you don’t have heat, that’s still a huge problem, and places like Texas that are realising that courtesy of climate change they now need to prepare for the cold will still need to do a lot of work. But, heating infrastructure is child’s play compared to the logistical nightmare that is cooling infrastructure.
Building managers/stratas/HOAs, at least for townhomes and apartments, still stubbornly disallow window air conditioners or mini splits, you know, the better and more energy efficient ones. Because god forbid the building looks a little cluttered while the residents are dying of heat stroke.