Well, the number a little misleading especially when used to bash people, as most homes not insured are in areas that are deemed safe from flooding. Each area is evaluated, and most areas do not require it. For instance, my area is deemed safe, and my mortgage company told me it was unnecessary. Many that may struggle with monthly bills may have opted out, because it would require something of epic proportions for it to be needed.
I still have it, because it is a low cost (maybe $400 a year?) for peace of mind. I have sympathy for those that opted out though.