Why are there no unique voiced kana?


If one were to look at the 万葉仮名 article on Wikipedia, they would find that no
specific marks were added to represent the bunch, that is, the ザダガバ 行, with
there having been different representations given to the sounds. However,
excepting those kanji, there does not seem to have been anything that could
represent those sounds per se.

Why would none of them ever be reduced to kana? Was there some factor that led
to this preference in using diacritic marks to distinguish them?

