I think it is because the cast iorn is more massive. It acts like a
flywheel, asorbing the shock and energy from the bumps in the road.An
energy sink.The sheet metal lower control arm ,though heavy, dosent seem to
absorb,or hold the energy. Maybe it even transmits energy to another
location , better than the cast iorn.Dont know. I am not a
physisist.However,having both types, my butt says  make the effort to get
cast iorn if you are going to spend all that energy and money on a front
end rebuild.The parts from a late vanagon will fit an early vanagon.You can
even rebuild at your leasure this way. Get the new parts and press in the
new stuff. Maybe even paint the parts to look like new. When you got that
done,swap them out.