The 24-70 f4 IS is canons IPAD MINI, they are targeting a specific crowd, amateurs. People who buy the 24-105 buy it for the is rather than the straight reach, and all the amateurs buying a 6d probably are wannabes portrait shooters, so they go for a 24-70, its a cheaper alternative for a 24-70, not a 24-105 replacement, canon isn't expecting everyone to love it.
I failed to understand your logic here: I am sure many amateurs buy it for the reach so they don't have to invest in another lens (105 is sufficient tele for many people). And 24-105 is a great portrait lens both on APS-C and FF, so I am not sure what all these 'amateurs' would have missed out if the 6D was shipped with a 24-105.
On the other hand, it makes sense that the 24-70 is targeted towards people who want to shoot macro, or weddings if the IQ is comparable to 24-70 at equivalent apertures (talking real-world, not MFTs). Would be interesting to see how it compares with the Tamron 24-70 f/2.8 VC though...