There are so many reasons, not just one, and I think everyone here has covered it.
*In Asia, having a large family despite a low income isn't something that is really frowned upon. I think Asian children in general behave better than others, are more manageable and therefore one can have many children without ripping their hair out.
*As many have said, there is the investment-child mentality — nobody will care for you when your old, so better raise some children that will. Also, having more children means having your bloodline carried on and spread out.
*Culturally, Asians prefer to be around very large families rather than a small family and many non-related friends.
*Contraception and sex-related discussions aren't big in Asia, and we all know that can only lead to an ass load of unplanned pregnancies. It isn't considered socially acceptable to openly talk anything sex-related openly or casually. Everyone likes sex, Asians like sex, but Asians are more of the 'I'll keep it to my self' type rather than some of us over here that prefer to proudly describe every detail to everyone we know, and many we don't know. I think when it comes to this point, they have a lot to learn from us and we have a lot to learn from them.
*Less diseases...you know, the type that wipes out half your country. In Asia if you get the chickenpox it's likely that you've got it from some white guy/girl at a mall. Asians don't do diseases. Things like getting the flu don't really happen due to the tropical weather, and STDs aren't that widespread as most people only have sex 1 person or maybe 2 in their lifetimes, and gay sex is out of the question.
*Governments....don't they suck. I can't think of a single one that isn't a pain in the ass. Anyway, most Asian governments don't push contraceptions forward, and none of them subsidise (i.e. make them free of charge, take as many as you like) the cost of condoms when given to younger users. Condoms also aren't something that's readily available, so even if you would use a condom you can't. Why? Because contraception and sex-related discussions aren't big in Asia (see point 4). It's a vicious cycle, and we all know that can once again....only lead to an ass load of unplanned pregnancies.
BTW has anyone ever been to countries in Asia? I've been to India, China, Indonesia, Philippines and Singapore and despite the huge population I never seem to see ANY pregnant women around, whereas in the UK preggers seem to be everywhere. Perhaps I just subconsciously ignore the prescience of pregnant Asians? :/