First off, the 6970 doesn't represent value for money in any way, shape or form. The 2GB 6950 isn't that much slower and costs a fair wedge less (and more than a few can be unlocked to 6970 spec anyway). Then of course bear in mind that for single monitor gaming, the 1GB 6950 offers near identical performance to the 2GB version. A quick check of the price difference between a 6970 and a 1GB 6950 makes for a sobering comparison.
Secondly -and this I was unaware of until I was emailed
a link a couple of hours ago- the HD 6970 doesn't seem to be a great seller for board partners. Allowing for similar numbers from the HD 6990 and GTX 590 (a fraction of sales), the HD 6970 doesn't seem to stack up well against the GTX 570/580 if you note the sales volumes in the $300+ bracket.
Maybe the lower sales (compared with the 6950 ) and the extra added value board partners need to incorporate into the 6970 ( as in "factory OC specials" -thicker pcb's, sturdier VRM's, more complex cooling) makes the 6970 non-viable. Factor in the initial designs heat/noise characteristics that made the reference models less palatable for many prospective customers (a lot of people held of buying until custom cooled cards emerged).
The only two other scenario's I can think of both would be longshots:
1. Not enough fully functional GPU's to keep the 6970 lines running (very unlikely given the success rate of "unlocking" 6950's)
2. Clearing inventory to make way for a HD 7xxx replacement ( unlikely IMO, as there would have been some benchmark leaks by now. I also think clearing inventory would affect 6950 stocks -which remain high)