There hasn't been much history for heat waves in northern Europe, but we used to have very long and cold winters. As such people invested heavily in heaters and insulation, and simply opened the window during the warmest few days during the summer. An AC unit and installation is quite a large cost in order to just make a few days better once every few years.

Now days the winter is much milder and the summer warmer. Modern ACs that can run in reverse is also much cheaper and can reduce the need for more expensive heating solutions, so we are seeing a uptick in number of homes with AC installed.