One reason to think it'll happen much quicker in trucking is because the economic incentive is much higher for a trucking company to replace it's drivers than for a single person to use an autonomous car.
Additionally, the safety and compliance standards are different. For instance, ISO 26262 only applies to passenger vehicles.
However, while the average consumer won't pay an extra $10-??k for a first generation self driving car that can't navigate their daily commute, a trucking company will glady pay an extra $50-???k for a first generation self driving truck that can only drive on highways to distribution centers if it means eliminating the human driver. The latter is as much as 50% of the operating cost of transportation, even when long haul trucking, so as long as capital and maintenance costs are comparable there is a lot more room to profit off of each sale. Even if it takes autonomous cars many more decades years to become an every day consumer item, autonomous trucks will likely be operational much sooner just because there is so much more money to be made in the short term from a few clients like Walmart, Amazon, Costco, etc. that don't need a perfect driving experience to benefit financially.