From what I've read here, saw in advertisements, and similar, I would expect that the reason for the popularity of the SuperSpeed was a combination of advertising, simplicity for the end user, and the implied additional safety of not having to risk playing with the edges of a blade when loading and unloading the razor. (The whole 'unwrapped blade in cartridge' where you didn't even have to put your fingers close to the edge to load it into the razor)