One big factor is it often comes down to which worker group has a stronger union and likewise entrenched political interests at local levels, as we've seen in technological progress elsewhere.

Second to that is probably the safety regulatory process. 'AI' or computer automation has also been used heavily in airplanes for a long time so that regulatory process is probably going to happen much more advanced than cars.

Finally, driving regulations often happen at a much more local level (cities can ban AI drivers in their borders for ex) but flights are divided over much wider geographical lines. And there are far fewer airline companies than truck companies to deal with.