If I had to guess, I'd wager the reason why we aren't hearing about hybrid-electric diesel trucks is due to the weight limits. Trucks in the US are normally limited to 80,000 pounds. The weight of the batteries and associated equipment for a hybrid truck would almost certainly be higher than a conventional truck, so unless hybrids get an increased weight limit, you'd have to reduce the payload. Reduced payload means the owner/operator would likely make less money per run unless the increased fuel efficiency were enough to make up for the reduced payload.

Perhaps an increased weight limit for hybrid-electric trucks would work. The increased fuel efficiency would have to be enough to offset the higher purchase price in a reasonable amount of time, but that might not be too hard. I've read that many truckers log as many as 100,000 miles a year, so an increase in fuel mileage of even 10% represents a pretty significant cost savings each year.

While looking for info for this reply, I did see mention of a company that fitted their tractor-trailers with electronic engine controls that doubled their fuel mileage. Perhaps efforts like that make hybrid-electric trucks unnecessary. However, it seems the military is interested in hybrid-electric technology. I've read of one effort to make a hybrid-electric Humvee. In that case, the electric power not only provides for propulsion, it can power a powerful battlefield laser.