Because they thought that marrying a girl would need a lot of money and spending it on education was pointless. they thought getting girls married as soon as possible was good
People thought that education of a girl child was burden because we had this custom in our country "dahje prata"(dowary) the grils parents were supposed to give the boy all soughts of gifts money and a never ending list.. so traditional parents thought give education for a girl child is a waste because they have to spend alot  of money i n her marriage and they will have to stay at home and do no other work then a house wife used to do