Cost. I’m born and raised in LA and now live in Miami. I’m also in real estate. Type I is significantly more expensive but is necessary in South Florida because of building codes changed to withstand hurricanes, specifically after Andrew in the 90’s. Smart change to the codes. CA theoretically has values that are high enough to justify the cost but it would decrease land value significantly and make development that much more difficult. A reasonable pivot to making new housing more fire proof is changing type V to type III modified. Type III modified is wood but not nearly as flammable as conventional type V.