Good question. I think they would have liked to call it RSD version 3, in other words just an upgrade of RSD2.5. The way this new version was announced (before launching) was so that some of us, me among them, thought we would have to learn everything over again, and we feared that we could not go on using/updating the projects we had created. So we protested, and the staff listened. Therefore we can keep RSD and SD installed on the same system, we can even still download the last RSD version. Quite good service, actually!
It is a pity though, that the names are so similar, and also, that projects created with either of them have the same file extension, .rsd. You really need to keep your tounge straight when working with them.