As far as electric goes I would imagine that it's is likely due to the fact they run all the time. It's a lot faster to fill them up with fuel than charge them. You would have to keep a spear or two just to rotate while they charge. And even though they don't go far in a single trip, I can see it adding up quickly enough to be an issue. But I bet they start getting replaced soon as the older vehicles reach their end of service life.

As for autonomous vehicles go you would think we would start with trains.