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.