Hahnemuhle photo rag is a cotton rag fine art paper with a matte surface. It works best with matte black ink, which has been designed to provide a better d-max on matte papers. Photo-black ink works better on gloss/semigloss/luster papers, where it provides a darker d-max than any matte paper/ink combination can.
I believe that the issue is the interaction between ink and paper. Gloss/luster papers permit the glossier, darker photo black ink to "set up" just beneath the smooth polymer surface, while matte & fine art cotton rag papers encourage the ink to "wick" deeply into the paper, killing the d-max. As I understand it, matte black inks have "grittier" pigment particles that resist wicking into the paper, at a cost of a more fragile surface and a d-max weaker than that of photo black on glossier stock.