They have to add more lightness. When you look at steel frames, they all look the same from the outside. Using high grade steel allows it to be made thinner, which means less steel, less weight. This pamphlet is from the 70s.
I'm kind of a Reynolds 531 junkie. It's the tubing that was the standard for building Tour de France bikes for half a century, from the late 1920s to the late 70s. Bikes made from 531 are even by todays standards top end stuff, with the weight savings of newer steel being negligible. Nothing has the history of Reynolds 531.
by jsp612