The vast majority of the people directly affected by the Morganza opening choose to live in fishing and hunting camps. For the most part, this part of Louisiana being flooded is uninhabited wetland.
http://fiveprime.org/hivemind/Tags/bayou,camp/Recent
These people make a living out in the wild and will return when the water goes down far enough.
A smaller number of homes in outlying incorporated areas such as Morgan City will also be affected, but these homes are being sandbagged. Again, these people know the risks and choose to live there.
It makes no sense at all to farm the blackest soil in the world, especially when it floods two or three times a century. It would probably make much more sense to build cities in that lowland instead.
For the same reason they are allowed to build a city below sea level.