With production tax credits, for every litre you produce, you get funding. In the U.S., it goes through the tax system, but it's repayable, and they have direct pay, so it's a direct government incentive for every litre you produce.
Investment tax credits are built around the amount of money that you've invested. They are not related to the cost of production. When you have fuels that cost more than the fossil fuels that they are replacing, because they have lower carbon intensity, they need that help to level the playing field.