Because people get excited about the possibilities of higher, stronger, faster, deeper... whether it is in the world of luxury watches, automobiles, aircraft or even japanese porn.
As a diver myself, I can attest to the fact that there is little need for recreational divers to go below 50m. Technical diving commands more advanced setups, though 300m would be pushing it. In any case, the (% of population that needs to go below 100meters) multiplied by (% population that feels the need to maintain wrist fashion while having to gauge depth and potential hazards from all three dimensions) is a low number.