> We expect our new fully driverless service to be very popular, and we’re thankful to our riders for their patience as we ramp up availability to serve demand.

I think this explains the reasoning behind the gradual ramp-up. Even at the product launch like new iPhones, the demand is difficult to estimate, so IMO it seems more responsible to give a reasonable expectation to the public like this, than just saying bravely "the service is fully public from today".

I think the take-away from this announcement is their commitment for wider rollout over time from this point on.