Because China is a male dominated society and, though the situtation is improving for women, the better options for life are still offered to men.

Boys will stay with the parents for their rest of their life while girls may leave with their husbands. something like that,it's complicated conception

It seems like having a daughter would be easier. Having a son means he's going to have to work hard just to hopefully find a good wife who will be satisfied with what he earns, etc. You have to invest so much to make sure some woman eventually finds him suitable. Apparently if you have a daughter all you really have to do is make sure she's pretty enough to get a rich enough man who will take care of her, you and his own parents. I'd opt for a daughter.

(Before Chinese people vote this post down, I'll have you know these statements are directly based on what many different Chinese acquaintances and older students have told me!)

different people have different views, the middle-class or someone have good enduction maybe thought that no matter bear boy or girl. but for old man especially  for some rural area, the boy still a good labour than girl.  and the boy would heritage the familiy name, that a big thing for chinese family

but now in chia first-class city, have a boy mean a heavy burden for family, when boy grow up ,as a man he need have house and car and saving before he marry a girl.  but the girl's family don't need pay so much propertys.

I agree with lchickman. Unless you are a poor farmer a girl is the best and easiest option. This has changed full circle from the early days of the one child policy where a boy was looked apon as the prefered choice in almost all demographics. Now as long as the girl is reasonably attractive all she has to do is wait until she finds one of the many eligable suitors and find one who can improve her situation( and her families). Of course with more wealth and better career options available then this is often not even a pre requisite any more and many can now choose a partner based on attraction only as their parents will be self sufficient.