There are a number of assumptions in your post that may or may not be true.
For example, quite a large number of those Katrina victims DID have insurance. But there were so many claims that a number of insurance companies just said they are not going to pay any more, and were willing to deal with the lawsuits in hopes they could beat it in court. They knew they could not afford to cover it outside of court.
Secondly, quite a large number BELIEVED they had insurance that covered flood damage. This generally stems from attempts to confuse people into buying more insurance than they need by the insurance companies, who put all kinds of things in legalese and fine print so they can get out of paying for a whole bunch of things later.
And then there were a number of people there who wanted to leave, but could not afford to do so. This is a major part of that whole fight that occurred that day--who was supposed to authorize the evacuations and National Guard and all that.
I would blame the Governor and Mayor Ray Nagin, who were most responsible for the National Guard and evacuations, respectively.
However, you do have a point and the federal government also failed them. It turns out that a large number of other countries were quite ready to help us when Katrina happened.
Many countries sent money, totaling over $400 billion, to help Katrina victims. Even China was among these.
Other countries that could not afford to send money instead chose to send food and medical supplies; estimated worth of those food and supplies being about $100 billion. Even Iran was among these.
And Greece sent two of its largest cruise ships, capable of handling 1200 families EACH.
What happened to all of this? The Bush Adminstration refused it. ALL OF IT. We quite literally could have helped every single person there without spending a dime.