It's their culture.
Up until fairly recently, people in a certain areas lived farther apart because it was cold and the environment could not sustain dense populations and things such as villages and towns were very rare on account of this. Being warm and friendly just strangers out in public was irrelevant to them, as there was nobody to be warm and friendly with except their own immediate family. And being stuck with them most of the time, well, being warm and friendly tends to wear a little thin.
Flash forward to the modern world and everybody's clustered together doesn't mean that a population is going to lose a type of behavior that has been theirs for tens of thousands of years.
As a rule of thumb, the warmer the climate, the warmer the people. If someone is not nice, they have a tendency to not last that long.
The converse would be true for people who live in colder climates.