Any answer to this must necessarily involve generalizations about such countries that may not be true for all countries. With that said, the most likely reason for differences in cultural values is the fact that high-income countries are more industrialized and modernized in their economies. This leads to different ways of life and different values.
Low-income countries tend to have more traditional economies. People do things in ways that are similar to the way in which things have always been done. People have likely not been exposed to very many influences such as education and travel that would have changed their values.
By contrast, high-income countries have undergone a great deal of change. People have moved from the country to the city, thus breaking up traditional patterns of family and community relationships. They have been exposed to people from many different areas through moving, through travel, and through things like education and entertainment. These changes make their culture much more cosmopolitan and "modern."
For these reasons, low-income countries tend to have more traditional cultural values than high-income countries do.