Two-off, actually. Or four-off, depending on how you look at it. 2009 will see four half-hour specials: a new two-part episode, a special about its making, and "a clip show with a serious difference."
I wonder how the BBC managed to get everyone back together at last.
Crap, seriously? Why even bother. They were ratings champions with season 8, why not a whole season?
I don't know specifically, but I suspect your questions might answer each other. Maybe the Beeb couldn't get the cast back together for a full season due to scheduling or money concerns, but were able to get them together long enough to do these specials.
Anyway, season 8 was nearly a decade ago. We can't assume the show is as well-remembered or would do nearly as well in the ratings today.
Besides, a full season of RD was only 6-8 episodes anyway, so two episodes and two specials aren't that bad.