I'm not a geologist, but my understanding of coral atoll formation is as follows: At some time in the geologic past, an underwater volcano grow from the ocean floor, and broke through the ocean's surface. The volcano goes extinct, and subsides. As it subsides, a fringing reef develops on its sides. So it seems to me that unlike the sea floor in coastal waters that gradually slopes to a continental slope, the side of the reef is very steep, built up on the side of an extinct volcano that is also steep. I think that steepness affects the ability of ships to anchor off the coral reefs.