If the market for it doesn't exist, Canon PR will make sure to create one.
Your opinion of Canon marketing is higher than mine.
By the way, I have heard 5DIII sales trump D800 sales, makes sense, but do you know for a fact that D800 sales have been lackluster?
Of course, true sales numbers are proprietary, so we can't be sure. At least in the US, we can get a little help from Amazon.com.
D800: #17 in Camera & Photo > Digital SLRs
D800E: #74 in Camera & Photo > Digital SLRs
5D3: #9 in Camera & Photo > Digital SLRs
5D3 kit (w/24-105): #13
So both distribution versions of the 5D3 are significantly higher than the D800 (which doesn't appear to have a kit version)
Lackluster is certainly relative, but if you consider these two to be direct competitors (as many seem to), the less-expensive D800, with better sensor, is selling at a significantly slower rate than the more expensive 5D3. If I'm Canon, I'm happy about this; if I'm Nikon, I'm not happy about this. I'd call that lackluster.