[theultdev]

same reason people don't have A/Cs in some places.

although you'll get heatwaves sometimes, it's not worth it overall.

similarly, although these may be more efficient than what you already have, it costs money upfront to install new stuff. money people don't have.

[dv_dt]

I sure wish heat pumps were higher in my awareness when I had to replace an old AC/Gas central air stack - though that was many years ago before the efficiencies of heat pumps were quite as nice as the current generation.