"Endogamic marriages (i.e., within the circle of one’s relatives) were preferred by ancient tribes. The chosen suitor for a girl was her cousin; it was actually forbidden for the eldest daughter to marry outside the family." (The Jewish Encyclopedia)
Since the human race was younger, birth defects not as likely
When Isaac needed a wife, they went to the family to find one; Esau had married Canaanite women, and the his parents hated it. So Jacob was following the ethic of the culture and probably hoped that Rachael would too!