"This one on frame design suggested that the main impetus behind the move to sloping top tubes is that one frame size can accommodate a much wider range of differently sized cyclists, so fewer sizes of any given model need to be produced. The ultimate purpose was to reduce production costs, which in the case of custom carbon-fiber would be several orders above astronomical - to the point of being impossible."
This is absolutely true. Giant sold the compact frame design to us dealers as a way to reduce inventory models by limited frame sizing available saying fewer sizes fit more riders. They sold it to the racing public as a lighter overall bike because the frames were smaller.