I wonder if their decision was driven by political savvy - self driving cars help the blind/elderly, and automate work most people are glad to see automated. Self-driving trucks are going to have a major employment impact which will likely draw political resistance.
The implications for self driving cars is nothing short of an existential threat for so many people in this world. As we know, it's just the beginning.
Similarly, Apple moved very slowly in introducing the iPad even though the under technology had been available for years. They started with the iPod, then advanced to the iPhone. At the time the iPhone caused a great amount of tribulation. People were very uncomfortable and large portions of the tech sector predicted it would be a monstrous failure. Their apprehension was cloaking a deep-seated unfirmiliarity (and maybe some fear). Once the iPad finally did arrive it was easily digested by the market. That was the mistake HP and windows made when they introduced the tablet in 2001. People had no concept of what a tablet is or how to integrate it in to their lives, so it was a miserable failure.
The lesson is don't move so fast that you scare your ultimate market, the people.
Apple did introduce the Newton, many years earlier. Their slow pace for iPad may just have been due to lessons learnt.