And the currents near Woods Hole are very strong, which would make construction even more difficult.
Actually, the water between Dauphin Island and the mainland is so damn shallow, it seems unlikely that a ferry of any practical size would be able to navigate it. So maybe a bridge was actually a cheaper option.
Map here.
It does appear to be a popular beach destination from the map, it is somewhat reminiscent of the Long Island NY barrier reef beaches connected by bridges and long causeways. Usually the route of the bridges are across shallow water or on marsh lands which is more of a causeway then what is considered a bridge, a bridge connotes an expensive spanning structure, while a causeway is just a roadway build over water.