Why no f/2.0 zooms? One for the optics experts.

The fastest aperture zooms for Canon (and Nikon) are f/2.8. What are the challenges preventing an f/2.0 zoom?

For example, a 24-70 f/2.0 would be in the middle of the focal length extremities and hence easiest to build.

I assume it would easier for EF-s, given the smaller image circle requirement, but they have not done that. Olympus have f/2.0 lenses for the Two-Thirds format.

--
http://www.dylanwalsh.net/photo/