In Canon's defence, the 24-70 is often used for photographing people/events. For this purpose I tend to prefer a shutter speed of 1/80th or above, anyway, which is sufficient to prevent camera shake at even the longest focal length on a full-frame body.

Also, there's no stopping Canon from producing a 24-70mm f/2.8 IS, too. Just look at the 70-200 range, there are 4 of them! - but how much do you think THAT would cost?