-Speculative buying (that's the cause of all the empty appartments in places no one wants to buy)
-Cultural reasons. When a guy wants to marry a girl, he has to own his own appartment. No when, no ifs, no but. If a girl were to marry a guy with no apartment her whole family would lose face. Because of the sex imbalance in China, there is a large "demand" of girls, and little "supply", so the "price" (quality, location and numbers of appartments) keeps rising. So this means parents of boys are constantly upping the ante, to get more and bigger appartments to try and ensure their son gets married. (my girlfriend is Chinese, so I am very aware of these issues...)
While the first reason would seem to indicate that the bubble would burst sooner rather than later, the second one seems to indicate that demand for appartment won't fall anytime soon.