I'm not sure about that but I do know that in areas where air quality is a major concern (not just fuel economy and CO2 emissions) there is some movement toward using gasoline engines in medium duty vehicles. There are actually versions of this V10 that are squeaky clean with respect to smog-forming emissions. No diesels can approach that.

Furthermore you have to consider when the development of this system was being done. The entire diesel industry is in flux, the Ford products switched from an older diesel to a newer diesel that was barely ready for mass sale when it hit the streets (and Ford's lawsuit against Navistar about the level of warranty issues with that engine suggests they were wise not to rush into adding further complexity to the finished product!). The people doing hybrid development needed to work on a stable engine platform so they could be assured that when they were ready to hit the market with their hybrid package, the engine had successfully integrated was still available. They couldn't do development of the hybrid package with the new diesel that wasn't ready yet, wouldn't want to do development on a diesel engine that was soon to be discontinued. The V10 was ready.

Finally, as Toyota and Honda are always reminding us, if you use hybridization to avoid running a gas engine under light load (under conditions where the diesel efficiency advantage is largest) the efficiency gap narrows quite a bit. And if you want customers to be able to pay for the hybrid components, it helps to not include the (huge) price increment to upgrade the E-450 to a cutting-edge diesel.

That said, if things stabilize on the diesel side of the family, it's reasonable that they could have development ready on that side soon too.