The reason they haven't done it is weight. Trucks are legally limited to 40 tons in most places in the US. All those batteries are heavy, which cuts down on the cargo you can haul, which cuts down on your pay, which cancels out the money you save on fuel.
Do expect to see hybrids popping up in local delivery trucks. Driving in the city with lots of starts and stops and usually at less than maximum load really helps the hybrid economics work out.