Unfortunately there is no real answer to that question. Some had no money (gas, hotel), some had no cars, some had their cars break down on their way out of town and no way to get them fixed, some wanted to stay with others who could not get out, some waited for friends/family to get home so they could leave together.
Watching the news some people were begging others for $20 to buy gas so they could leave. Yes, sad as it is, there are people that indigent.
Was it three days notice? I did not watch the news that closely beforehand, I think Katrina built into the power that she was about 1-2 days beforehand in the gulf.
I was in a hurricane (Floyd - 1999) and there were no evacuation orders in the suburbs of PHL. Only thing predicted was high winds. I woke up in the morning with no warnings on the news, only heavy rain. Business as usual as far as the news was concerned. Within three short hours I was driving down the street as the water was rising with only a few bags stuffed with clothes, some papers, some photo albums, some food, and two cats screaming in the back seat as the torrential rain caused the water from the tiny creek across the street to envelop the area.
Sometimes nature's fury can just happen so fast and be so powerful.
Mudkow, thank you for your help, you and everyone affected will be in my prayers.