AlanF said:
Sigma and Tamron make f/6.3 lenses that focus on all Canons, and Canon has just introduced improved f/8 lens focussing. So there is no AF compatibility reason against f/6.3.
Well, no reason other than Canon says a max aperture of f/5.6 is required for AF on many of their cameras. But that's a pretty significant reason. Back in the day, you could spend a few thousand on a 500mm f/4.5L USM, but add a Canon 1.4x TC for a 700mm f/6.3 lens and you'd lose AF except on the EOS 1v and EOS 3 (the only f/8 AF bodies at the time).
For Canon to release an f/6.3 native max lens for dSLRs, they'd either have to update all bodies down to the lowest Rebel with f/8 AF, or state 'we're big fat silly-face liars and have been for years'. I expect the former is more likely than the latter, but either way we're not going to see an f/6.3 lens from Canon any time soon.