Why did Wang Lung and O-lan fall on hard times in The Good Earth?

In The Good Earth, Wang Lung and
O-lan do not fall on hard times through any fault of their own.  The farmers in Wang
Lung's area are greatly affected by drought, and they cannot sustain themselves through
their land when the crops fail.  Because Wang Lung and O-lan have been frugal, they are
able to last much longer than many others in the area, but eventually even they must
leave their farm behind to travel to the southern city in search of money and food.  At
this point, the couple begin to see their children as a burden because they feel guilty
about not being able to properly provide for them.  O-lan suggests selling their
daughter into slavery; however, Wang Lung cannot bring himself to do this to the
girl.