[m1el]

It's not about being poor. First, the climate didn't require AC in most of the Europe, until ~10 years ago. You had a few hot days, and that's it. Second, thermal isolation in the US is extremely bad quality. I think people could cut their AC usage by half if they had proper thermal isolation in their houses. Third, northern Europe countries still don't have a climate to justify buying an AC.

[brnt]

Specifically, American houses lack thermal mass due to being constructed mainly from wood. Concrete and brick will buffer over a week or so of heat before it warms up too much.

[firesteelrain]

In Florida, most of the homes are built from concrete brick with wood trusses. There are apartments made from wood and concrete.

It’s not the heat completely - it is also the humidity. You can bear up to 80 F before it starts to feel uncomfortable. Humidity will make even 75F uncomfortable.

[rayiner]

Thermal mass doesn't matter much because the air in a typical home is replaced every hour.