The usual way to reclaim land is to pile sand rock on to the seabed. When the seabed oozes with mud, this is rather like placing a textbook on a wet sponge: the weight squeezes the water out, causing both water and sponge to settle lower. The settlement is rarely even: different parts sink at different rates. So buildings, pipes, roads and so on tend to buckle and crack. You can engineer around these problems, or you can engineer them out. Kansai took the first approach; Chek Lap Kok is taking the second.
The differences are both political and geological. Kansai was supposed to be built just one kilometer offshore, where the seabed is quite solid. Fishermen protested, and the site was shifted a further five kilometers. That put it in deeper water (around 20 meters) and above a seabed that consisted of 20 meters of soft alluvial silt and mud deposits. Worse, below it was a not-very- firm glacial deposit hundreds of meters thick.
even sink
buckle