For the easiness of lens design would be one of the big reasons, but another big reason should be the aperture control during the video shooting which is too crude or simply impossible on the current Nikon bodies.
One concern is that the frame rate of Canon is known to slow down when aperture is stopped down quite far, owing to the speed of the electric aperture actuator, especially on the big telephoto lenses and fast lenses, because the aperture blades have to travel longer.
That said, you rarely stop down to, say, f16 to shoot sports or birds. And Nikon should already have solved this technical problem, which may be why they finally went for "E" on the super teles.