24-70 f/4 from Canon makes sence.
In terms law of physics, the dimension of front element can be similar to EF 24-85 f/3,5-4,5. The whole lens and its optical formula can be close to 24-85, too or it can be even smaller.
The 24-70 f/2,8 mkII or 24-105 f/4 can be expensive kit lens for 5DmkIII. But they are too expensive to relative cheap 6D body. Canon needs cheaper kit standard zoom for 6D. So the Kit 6D + something can have relative lower price opposite to 24-70 f/2,8 or 24-105 f/4. More people will buy 6D kit with this lens then.
Orientation to video (like EF 24 IS, EF 28 IS, EF 40 STM etc.) is good reason, too.