Some of the research I have done has shown the following reasons. The World is shrinking. Everyday we meet people from other countries. Each has a different language, customs and cultures When everyone is thrown into the same pot we find as many different behaviors as there are people. The reasonable behavior of one person may be in direct contrast to the reasonable behavior of another.
People are tribal.
We find more rudeness in groups of like people. On a recent trip to Egypt I observed tour groups from different countries kept to themselves, even though we all shared the same facilities on the ship. The Italian Tribe stayed together. The Japanese tribe and the Swiss tribes stayed together. Only when someone was separated from their tribe would they speak to someone outside their tribe.