Same principle as why many people prefer not to own shares in the company that employs them -- you're already heavily exposed to that specific risk and don't want to add more. If you live in Florida then a hurricane in Florida already might mean financial loss for you if it damages your house, so buying a CAT bond that covers a different thing is more diversified risk: you might get "house is trashed" or "bond is total loss" but at least you probably will not get both at once.