Probably for 2 reasons. With the west and east freeways there are local roads that can be used by those who have to travel inbound(such as out of state relatives rushing in to evacuate a shutin parent) but not so for the causeway, which crosses the middle of the lake with no options for 20 miles either way. Also, the causeway ends at an east-west freeway (I-12) on the north shore, with no northbound freeway to continue on, just some 2-lane roads.
Thanks! I wondered.