Good question...and I don't know, not explained to me...if I was to guess and please chime in here, could it have something to do with the EF and the communication between cameras and lenses? or DPAF and how it works? I could easily see Canon not wanting to compromise ANY of the auto focus functions they have worked so hard to create. The DPAF does have a whole second set of pixels so it "triangulates" the distance to the object so to speak.....again, I'm guessing here.
Jim Martin
EVSonline.com
It probably is due to the lack of DPAF on the GS version, on top of the fact that they assume the GS version will find itself being used on larger scale projects with specific needs for it. Arri don't even have a GS camera.