That's true, but diesel engines are only really less polluting in terms of CO2 due to using a lower amount of fuel consumed. If your goal is to reduce that, then as you say biofuels are the right answer.

However any mass adoption to biofuels would require different engine (existing engines will work fine on a low blend of biofuels, but very few will work on 10P%). For cars, why would you stick with a biodiesel engine which is more complicated and has higher levels of other pollutants, than some sort of biogasoline engine?

Engines running on biofuels equivalent to benzine are still significantly less efficient than diesels. They also require significant modifications, whereas diesels run on vegetable oils with little to no change.

Hemp oil powering diesels is possibly the most efficient locomotion path we have, only really competing with electricity generated with little carbon emissions.