Female children can be a burden in that a dowry is expected to be paid at a marriage in certain cultures and males might be expected to support the parents in c

Female children can be a burden in that a dowry is expected to be paid at a marriage in certain cultures and males might be expected to support the parents in certain cultures for another source of preference. There is also the culture that might prefer lineage to be passed down via males and some cultures prefer a male child to help in rural area manual labour around the home. These are mentioned in the article a bit - add to that the ability to determine the gender of a baby before birth and the ability to perform an abortion after determining gender and while technically against the rules in both nations, a gender based abortion will help create the gender imbalance. There's also a natural slight skew to having male babies among humans, it's much closer to 50/50 on conception but by birth it's not due to higher female mortality in the womb.