Trucks are big, heavy, and human-driven ones already scare enough people on the road. If you launch with trucks first, you risk a backlash from people who are afraid of them. If you start with an autonomous taxi service people can sit in the cars themselves, feel safe, see that there's a "safety driver" present, and even if they aren't in the vehicle they still get to interact with them on the roads. you can make people comfortable with the idea before you launch the trucks. and everything that goes into the self-driving taxis is still valuable development work for the trucks. I wouldn't be surprised if self-driving trucks actually are the first real commercial launch target for waymo, and the taxi thing is just PR.