It's expensive to evacuate. First, you lose money because you're not working. The extra fill up, or two of gas is well over a hundred dollars. The hotel room, if you can find one, is another unplanned expense. Food is expensive.
Another thing: Unless you live a rural area, the authorities may not let you return home. Now, you not only have the evacuation expense, there's the added expense of living away from home, if you have such a place and can afford it. Your life is in limbo. You can't survey the damage, start the process of repairing, and the authorities really don't care how hard it is. They have theirs and the taxpayers pay for it.
So, some people don't evacuate, and they're most likely the ones that will need help when it's all over. To make matters worse, the current economy created more of the people that really just can't evacuate. So, you can add something else to the fiasco of the current administration. They not only created a horrible economic situation, they've given people few choices at a time when choices are important.