Re: Finally.
The Progress could be sent to orbit through a Soyuz launcher, but the same launcher could not take the platform as well, so , yes, the Shuttle could take the astronomy platform and astronauts could attach both of them, once docked to the station.
And of course, as Phil says, there's the cost factor...but it's a Scandinavian-Russian idea, so it would involve their willingness to finance it.
Raoul