Let me walk you through the facts here.
The poverty line for a family of four is $20,444 per year. That's $393 a week, before taxes, rent, food, etc. Around 100,000 people displaced by Katrina were below the poverty line (
source), so one would assume a sizable portion of New Orleans was making less than $400 a week.
Assuming, say, an evacuation to Baton Rouge, which was closer than most, evacuees would travel around eighty miles one way, spending about $30 on gas for a round trip- assuming they even had a car. Riding the bus is more expensive. A hotel room for four costs at least $50 a night (usually more, you know), and considering a stay of at least two nights, a hotel stay would cost at least $120 with taxes.
That's $150 just to evacuate and find a place to stay. Do you seriously think that a family can easily afford to make such a sudden payment when they make less than $400 a week? If so, you are hopelessly disillusioned.