One could argue that it's safer to transport the materials without the added attention from armored escorts, but they're probably just avoiding the added costs.

They may actually be right to, though. The costs of an armored transport is higher than a regular truck transport by an order of magnitude. The costs of calling in the cavalry that one in a million time that the truck is hijacked is probably far lower than securing each transport.

That doesn't mean they shouldn't be using lojacks and panic buttons for the drivers though.