Don't get me wrong, I'm not saying the 5970 is not a beast... I'm just saying that it's two generations behind and it has to be pretty difficulty to try testing everything from previous generation let alone including the generation before. Look at my sig and you'll see I'm running a pair of 5970s in my current rig... having had the card since it came out I know about where it lies in the charts (one 5970 is roughly equivalent or a little faster than a GTX 580 except on TWIMTBP titles) and then I have to take into account slightly poorer scaling with a pair of 5970 vs a pair of 580. I'd like to assume I have performance right around a 6990 or a little faster for the games that will use all 4 GPUs. Also, those with 5970s already know the limitation imposed by having only 1 GB of VRAM per GPU so anytime the resolution gets higher than 2560 or with high AA + fairly high resolution we'll see a pronounced decline in performance. Highly tessellated scenes will do the same.
Again, I'm not saying the 5970 isn't fast, just that it's been out for quite some time and the people who own them should have a really good idea where they lie in the charts anyway. Also, the number of people who own them is much lower than any of the other 5xxx cards so it wouldn't make sense to include the 5970 over those.