Gender Inequality:
Women are thought of as the lesser sex. Women are seen as "burdens" for the entire family, so it is often easier for the family if the girls are married off at a young age. The girls are now a "burden" for their husband's family, not their family.

Culture/Tradition:

This has been a tradition for many years. With tradition, there is usually little question as to why it started and why it's still happening. Unfortunately, this tradition has some even worse practices: female genital mutilation. Once girls begin their menstrual cycles, they are believed to be of age for marriage.

Poverty:

More than 50% of these girls in developing countries are living in extremely poor families. These families have to pay dowries when they marry their daughters off. These dowries are less expensive if their daughters are young and for an impoverished family, this is much more enticing, unfortunately.

Safety:

Women are much more likely to be harassed and subject to violence in these areas. Families often feel that they will be safer if they are married and under the protection of a husband.
(Brides, G. N. (2002))