I tried to google, but only semi-decent answer was that ECF didn't work for some and was a nightmare for support group.
But are there any reasons modern Canons (or other major camera-makers) lack eye-controlled focus (ECF) system?
I agree that was an awesome technology but only to the beginners I suppose. And even thought it was very popular among a mass crowd it had a few setback:
So Canon found that a lot of users don't need this tech, and even though it seemed as the future of cameras Canon ruled it out of their latest DSLR cameras.
I think the introduction of the joystick is really making a difference, too.
With plenty of AF points, it's a bit tedious to use both dials to select a particular one. The ECF can select horizontally and vertically at the same time. It can be considered a 2D input and is in principle superior to using dials.
DSLRs (some models) have a joystick which serves the same purpose. Given the choice between a joystick and ECF I prefer the joystick, because it does not have the disadvantages mentioned by Arun Babu in his answer. The ECF did not work very reliably for me.