Why did people evacuate Houston, but not New Orleans?

Have you heard the saying "Once bitten twice shy?" The people in New Orleans either thought they've seen enough hurricanes they can handle this one, or just weren't able to evacuate. Now that Bush has a MAJOR embarassment on his hands, he's not taking any chances in Houston, because, as said, it's the 4th largest city in the U.S. and the death toll if this didn't happened could potentially be astronomical.