Cost and competition with Nikon. If the street price of the 24-70 f/4 IS ($849 reported by CR-which will be the least expensive "L" zoom lens in Canon's lineup) is accurate, it's list price is $300 less than the 24-105 f/4 ($1,149 on Canon's website). Typically the 24-105 is ~$800 in kits with other cameras. Assuming that the ~$350 "kit" discount translates to the new lens, Canon's kits just got about $300 cheaper with a versatile lens, both in terms of focal length and near macro ability.

So, in competing against the D600 (whose kit includes the the 24-85 f/3.5-4.5 VR for $2,700) Canon can now offer the 6D with the 24-70 f/4 IS for ~$2,600 (6D base of $2,100 plus $849 minus the $349 "kit" discount). Canon now wins on cost, constant aperture and near macro ability.

Regarding the 35 f/2 IS. That could be a tempting lens for me. I am excited to see the MTF chart.