In ancient cultures, males were typically at least as valuable as females as the parents aged and could no longer farm/hunt/whatever for themselves. Children were bound to their parents by bonds of filial piety, but menchildren in particular remained part of their parents' family and were responsible to the head of the family--their parents. This was not only because men-children were traditionally occupied in bringing in new wealth, but because they stayed in the family forever.

While girl-children might be able to maintain bonds of contact and affection with their parents (not always so by any means!) they were usually no longer part of the parents' family after marriage. The only exceptions were matrilineal societies, where families were defined according to descent through the female line. In China (for example), which has been traditionally both patriarchal and patrilineal, the only hope for support of parents in old age was from their sons. They not only had to send their daughters out of the family but had to pay money (a dowry) for the privilege!

This seems to be why female infanticide has historically been rather common in China, whether or not it had the blessing of the state or of society. It does not have such sanction in contemporary China, because there's already a shortage of marriageable girls--due mostly to female infanticide and sex-selective abortion, because of the State's draconian one-child-per-family law.

China, of course, is not unique in any of these respects. Infanticide was also practiced in ancient Greece and Rome, though it was perhaps less often directed specifically at female children. Rome, in particular, had the same problem of woman-children being lost to the family (along with a substantial sum of money as dowry) upon marriage. The Romans solved this by modifying the institution of marriage (in practice though not in law) in about the First or Second Century BC.