I think fly fishing is more tradition oriented. Most fly rods are more conservative in design with less bling than conventional tackle. Also, fly fishing is more finesse oriented and tends to be closer to the fish. In this case, the extra bling can spook fish. Other than that, I can't think of any reason.

On a personal note, I don't put a lot of bling in my rods. I think a lot of builders add it because they think it makes the rod "look" custom. A clothier once told be to never buy clothes that had metallic thread in the label. The thought being that if the maker was trying to hook you with a fancy label, it must be because the item itself was deficient. I don't really know how that applies, but it always stuck in my mind when I was designing a build.