There are couple of reasons people do that .
1 . I think the most important one is that they attribute the strength of the family to stay united . Their concept of staying united is to keep on marrying inside the family and all the family will stay together .
2 . Marrying inside the family also help in preventing further division of wealth . Like traditionally people had land that gets smaller and smaller if they marry outside the family .
3 . It is hard to trust people outside the family . Parents feel more secure in marrying their daughters to someone from inside the family , as they think that they will treat her better . Another think related to it is the family influence on the relationship between husband and wife .
I think these are few reason why people marry in their family . But as time is changing so is this tradition .