Remember (or learn, if you haven't heard this before) that "insulator" is a
relative term. There's no perfect insulation, only degrees of insulation or conductivity.
If you were taught that insulation keeps you safe from electricity, you were taught a
dangerous misconception.
Water, for example, is an excellent insulator. Yep, that's what I said. WATER.
Pure water, that is. Put any kind of chemicals in it and it becomes a good conductor. Of course, it's hard to find PURE water anywhere. Even rain has chemicals in it. But in a lab, with properly purified water, you can prove that water is a poor conductor, which is exactly the same as saying it's a good insulator.
That's not much help when you are standing in a puddle and someone drops a live wire on you!
Rubber is another good insulator. Carbon is a pretty good conductor. What makes tires black?
Carbon. It's put in to make the rubber harder.
Still, tires are a pretty good insulator at lower voltages. And when you are talking about insulation, you have to consider the voltage. A material which insulates well at some voltage may become a good conductor at high voltages. Where that changes is called the 'breakdown voltage'.
I suspect those lines are a bit above the breakdown voltage for tires, especially tires that may have varying amounts of dirt & grease on them that might provide a bit of a path to ground.
At the kind of voltages that killed that truck, well, tires aren't much help.