I don't know, but I know something of metallurgy.
CGI is essentially "poor man's" ductile (nodular) cast iron. It's less brittle than regular grey cast iron, but not as ductile as true ductile cast iron. It's inbetween.
Using a more ductile material increases fracture toughness = resistance to crack propagation = you can use less ductile iron in place of grey iron.
Now you see why. Cost and weight reduction. Residential sewer pipes are made from ductile nodular cast iron so they won't crack due to ground temperature fluctuations, fyi.