Is there any particular reason why space-frame bike designs like the ones Moulton use haven't become more common? Is the actual design of the frame in some way inferior to a traditional diamond-frame? Moulton could use diamond frames like this http://www.flickr.com/photos/sashae/2832307116/ for some of their small-wheel bikes, but they don't, so the space-frame must have something going for it. Obviously, there's more welding/brazing involved in producing them, so I'd imagine they work out at more expensive than conventional frames, but couldn't a well-designed space frame be potentially much stiffer than a diamond frame, or the same stiffness for less weight?
Anyone have any thoughts on this?