P1. Causes have to have to be fully determinate, fully specific entities.
P2. Reasons cannot be fully determinate, fully specific entities.
C. Reasons cannot be causes.
The motivation for P1 might initially seem a bit obscure since some authors (e.g., Mellor) have argued that facts can be causes. Remember, however, that we're looking at an argument that's an argument against those who have already decided that facts cannot be causes, presumably on the grounds that they are not concrete.
As for the motivation for P2, the idea is that the rational role of reasons requires an entity that's graspable. Facts, propositions, and states by virtue of the fact that their natures seem to be determined entirely and exhaustively by the way they are canonically picked out linguistically. Whereas causes have to satisfy Steward's 'secret life requirement', reasons cannot have secret lives upon pain of failing to be a graspable.
This is all back of the envelope stuff, but I think it's a start.