The main reason Canon does anything is in reaction to what the competition does.
Ok...but, how does that explain f/1.2 lenses, the MP-E 65mm, the TS-E 17mm, the 800/5.6 which Nikon is only now apparently getting around to releasing, etc.?
Those are secondary reasons.
They have a technical advantage so they push it.
IIRC Nikon cannot offer 1.2 lenses with AF due to the flange distance of the F mount. A drawback in having to support legacy technology.
In terms of lenses Canon has a larger lineup but they only update in reaction to what the competition is doing.
Like say the 135L will probably get a refresh within 24 months while the 200/2.8, 300/4L IS, 400/4 DO 400/5.6L, 70-200/4L, etc will get one 24 months after today. The those lenses are rather unique to Canon.
35/1.4L is a certainty.
It just a matter of time.
Seeming we all know this will happen allows us the opportunity to set aside money for them when they come out.