Why do demographic and psychographic strategies fit together?
I assume that you are talking about demographic and psychographic strategies of market segmentation.
Psychographic segmentation is the technique of looking at the personalities of potential customers to determine if they are likely to want your product. Demographic segmentation is when you look at the potential customers' objective characteristics -- their age, their sex, their income, things like that.
These two strategies go together because the demographic characteristics can often correlate with the psychographic ones. It is possible that people in a certain age range, or those of a certain income range, will have similar interests and attitudes and that these attitudes will be different from those of other demographic groups.
So these two strategies go together because the characteristics uncovered by the strategies can be correlated.