Is this not obvious?

I was reading this article about how Toyota is working to cut the manufacturing costs of building hybrid vehicles so they can lower the price and how they hope that by 2020 all of their cars will be hybrids. What caught my attention was the first paragraph:

Toyota has always backed its hybrid technology as the key to reducing fuel consumption and carbon emissions, but as advanced as the technology is, diesel powered cars manage to offer similar levels of eco-friendliness but with more performance to boot. It doesn’t help that hybrid vehicles are considerably more expensive than their oil-burning counterparts as well.

Why the heck aren’t they making diesel hybrids? If a diesel engine gets as good of gas mileage as a hybrid, how much better would a diesel/electric hybrid be?