You can get double-sided DVD-RAM disks which provide the same space, actually a little more, I believe, at 9.x gigs.
Apparantly it was decided the DVD-RW DL media was going to be too expensive to produce for such a limited market. The price of such media would be more per gig than another hdd so why not just put it on a hdd? It is faster, more portable, and more rewriteable. For archival purposes, probably just as reliable: write it to the drive and store the drive until you need the material or you have to migrate it to a new format ...
There are Blu-Ray DL RW disks ...