In urban areas, at the level of units or buildings, aren't improved regulations leading to better fire containment, and/or improved firefighting response and techniques, likely to be contributors as well? There are still apartment fires in Manhattan and Chicago, but they seem much more contained than in the past, with whole-building and especially multi-block conflagrations now quite rare.
I am not sure on that point this reality didn't affect the area I was working on. What I do know is that the number of fires has severely decreased (nearly halved just in the last 5 years). If the number of fires, the ammount claimed per fire has increased dramatically, part of that is the price of building material, work hours and just the value of the insured assets. But it doesn't seem that having fire retardant makes fires any smaller.