Why are descriptive research methods limited in their ability to explain behavior?

Descriptive research methods are used to describe and observe behavior. Examples of these are Case Studies, Survey Research, Naturalistic Observation, Psychological Tests. They are limited to explain behavior because the descriptions and observations derived are based only on the current thoughts, feelings or behavior of a small set of individuals.

Americans often speak of their country as one of several large regions.  these regions are cultural units rather than governmental units --  formed by history and geography and shaped by the economics, literature and folkways that all the parts of a region share. what makes one region different from another? a region's multicultural heritage as well as distinct demographic characteristics like age and occupation make regions different and special. within several regions, language is used differently and there are strong dialects. there are also differences in outlook and attitude based on geography.