I noticed when I bought my DD-7 back in Jan/Feb that it was missing from the brochure in the box but still on the website. I had 1 but sold it when the 7 came along.
the DD-5 was already hard to find (here, anyway, and it was heavily overpriced) when the DD-6 came into being.. it might be that the 6's have been out of production for some time, and they're just finally unloading their excess stock (like the VB-2, produced for only a short time, but lots of unsold stock).
Boss probably won't skip any number except new "1" series pedals, and the unlucky "4".