Just a guess, but i'd say because they're expensive to make, and plenty of people are buying the fiberglass ones.
They probably figure most of the dudes buying the repro fenders are the dudes making "rodded" trucks.. usually creampuff weekend nice day drivers. If your rodding a truck and just driving it to weekend shows, fiberglass fenders are probably just fine, since you wouldn't need the durability of thick steel, and if your rodding a truck, your probably not overly concerned with originality.. in fact, the rodders might actually PREFER the fiberglass ones because of weight.
Steel fenders would probably require more expensive equimpent to stamp out than a fiberglass mold would.. factor in that alot of the repro panels are made overseas.
On at least two occasions i've seen steel repro fenders for the 41' - '46 trucks, and they were thin.. not sure where they came from, but some 175lbs dude sat on one at a show and left a nice dent about the size of his arse on it.
Just be thankful the repro BUMPERS are steel!
