I see two reasons for that.
Firstly, people simply do not know how to keep their houses cool. On hot nights, people will do the right thing and open windows to cool down the house. However, at day, they will not close windows and curtains to keep the cold air in and the heat out (opening windows will not cool your house if the air outside is warmer than the air inside, as it lets in warmer air)
Secondly, some houses simply are poorly designed for hot summers. Windows on the south are great most of the time, but for the (few) hot summer days, you need something that prevents the sun from shining directly into your room. A simple solution is to have a balcony; that takes away the sun when it is high in the air, but not when it is low, as in spring, autumn and winter.
(this assumes houses with 'real' walls that have some heat capacity. I do not think the almost-cardboard style walls so common in the US fit that bill. That may be a reason air conditioning is so popular there)
Is it internal, bodily factors or external factors (e.g. the house absorbing the heat? I don't buy that, though, as I can have all the windows open blowing cool air in and still not sleep).
There are probably some biological factors (still, I feel colder when sleeping, but of course I have a bad time sleeping when it's hot)
"When you sleep, your body’s internal temperature actually drops to its lowest level, generally about four hours after you fall asleep. Scientists believe a cooler bedroom may therefore be most conducive to sleep, since it mimics your body’s natural temperature drop."