Why is it lead(II) chromate and not lead(II) chromate(VI) tetraoxide?
Chromate ($\ce{CrO4^2-}$)is a poly atomic ion which is partially defined by the oxidation state of chromium.  Its like the sulfate ($\ce{SO4^2-}$), phosphate ($\ce{PO4^3-}$), nitrate ($\ce{NO3-}$), permanganate($\ce{MnO4-}$), and carbonate ($\ce{CO3-}$) polyatomic ions. For each ion the oxidation state of the center ion is defined therefore there is no reason to state it.  You could in principle call lead (ii) chromate lead(ii) chromium(vi) oxide or tetroxide, but this is less informative of the structure of the dissolved species.