The female infanticide is a primarily rural, uneducated, peasant phenomena that is fiercely opposed by the Chinese government, but old practices die hard.
Fortunately they appear to die with education--urban Chinese appear to prefer female children as they are perceived as staying closer to the family (much as in the West).
Note that India has a worse problem than China: The greater levels of poverty and lower levels of education in India have resulted in a ratio of 1.12 males per female, versus the ratio of 1.11 in China. The worst in the world is Armenia at 1.15.
There are also a host of European countries on this black list: Ireland, Italy, Spain, and Portugal, all with ratios of around 1.07 or so. Whoops but Western democracies aren't supposed to suffer from female infanticide.... uh oh, better sweep that one under the rug.