I would venture to guess that they intended for those textures as a gem sink, but realized that adding more gems to meet the demand would give crafting far less Metal residue as a result.
I also think they added the tier upgrade formulas after those BP's were conceived, which also would have left little room for even more demand on gems.
I could see though that as the demand for gems drops (more UL get tiered to 10), that it could trigger those bps to drop in the future, by reducing demand enough to warrant a new supply drain.
I tend to think everything drops based on formula, it could be that the formula just doesn't have the right parameters yet, and when it gets to a certain point in the formula, it triggers the possibility of particular drops.
The metallic BP even I think could drop again as well, provided we get enough people playing, and enough demand for body work in-game. It is just highly unlikely this game base, or body sculpting would ever garner enough attention or demand to trigger another.
My thoughts anyhow.