People won't subscribe at the real price.

Perhaps developers should have to bond new housing against floods prior to construction. Fully funded, third party bonds. Sensible locations will more or less be free.

Or that the existing models are not working

The reason why it’s not possible is that property/casualty insurance ordinarily spreads risk across a broad base whose risk is mostly indistinguishable from one another, and where the likelihood of claim is relatively low. The problem is that once you look at a flood plain map it’s easy to see whether you need flood insurance or not. And if the only people who buy the insurance are the highest risks, it makes little sense to sell it.