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?
1DX, 5D3, 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, 1.4x III, 2x III, 600RT x 4
The grass is always greener when you crank up the saturation in photoshop