Because it's the one we built all our cities in? It seems like an obvious answer: everything currently is built around current water levels +/- a meter or two.

Because it's the one we built all our cities in? It seems like an obvious answer: everything currently is built around current water levels +/- a meter or two. 10m extra of water does really bad things to a lot of costal cities. 10m less kills off a lot of shipping harbors and logistical networks.