My guess is that the bigger engine + servotronic instead of hydraulic just doesn't leave enough room for the 8-speed in the current build.
But like some others, I'd rather have the 6-speed anyhow. I don't like how the 8-speed jerks when braking and stopping, and it's easier to engine brake with fewer gears. And, if Michael Schumacher and Sebastian Vettel can do with less than 8 gears, so can I