Because when ice melts it turns to water. When lots of ice (the arctic) melts, it turns to water (the ocean). The problem is not only does this raise the sea level (effectively causing the coast to recede inward) but it causes more common and powerful natural disasters which, in turn, wreak havoc on specific parts of the country.

Which states typically face the worst natural disasters? Florida (hurricanes) and California (wildfires). When somebody’s house gets blown or burned away, insurance is supposed to cover the cost. But what happens when the insurance company spends more on paying out claims than it brings in in revenue? It goes out of business.

To avoid going out of business, these insurance companies are looking at market projections that use data attempting to predict future risks, or future likelyhood that they will have to pay out to their clients. Since climate change is only going to make natural disasters more severe, but ALSO more common, the companies are (intelligently) no longer pursuing business sin these states because it they are going to pay out more than they take in. If they stay, they would lose money.

Edit: “Wreak” havoc, not “Reek”.