I don't have an arsenal with which to defend the universalizeability thing; I don't use it, as I said. Kant seems to me to think that performing only universalizeable actions is a constraint on rationality; don't ask me how he got to that - if I had to use a CI formulation I'd go with the "treat people as ends in themselves" one.

But why this particular picture of morality?

It suits some intuitions very nicely. If it doesn't suit yours, fine; I just want people to stop trying to cram mine into boxes that are the wrong shape.