If you live on an island, it is difficult for you to travel and for people to travel to you. Therefore, because of decreased interaction, you tend to have a different culture than other peoples might.
Secondly, the leaders need to protect their authority. So, they put up artificial barriers (whether physical, moral, legal, etc.) to make it more difficult for the culture to change and be more accepting of other cultures.
As the world gets "smaller", cultures tend to intermingle. This is why, in San Diego, I can walk from a Vietnamese restaurant to a Mexican Restaurant to a Thai Restaurant to a Filipino restaurant, etc, etc, etc without having to travel more than a couple of blocks.
thank you!