The most obvious reason would be that Canon doesn't make it
but could this just be a marketing thing? Or maybe such a lens would sit close to the 300 f/2.8 II in price which if combined with a 1.4x TC makes an excellent 420mm f/4?
5D3, 6D, 600D, RX100
16-35L, 24-70L II, 70-200L II, 100-400L, 50L, 85L II, 135L, 24TSE, 40, 100 macro, 18-55 II, 55-250 II, 600RT x 4
I come here to learn something new, not to learn how bad my gear is - I know that already ;-)!