Sigma makes 19 f/2.8 and 30 f/2.8 lenses for Micro-4/3 and Sony E-mount lenses and just announced a 60 f/2.8 lens. The Canon M-mount uses the same 18mm flange distance as the E-mount and a sensor size between M43 and E-mount. The lens protocol is almost certainly very similar to the protocol used for EF and EF-S lenses. So why haven't they introduced these lenses for Canon?
Any guesses? Oh, by the way, I just e-mailed Sigma with the same question. I'll let you know if/when they reply.