^
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Actually, we do sometimes use the Greek prefixes when naming compound with metals, for instance, Na2HPO4 is disodium hydrogen phosphate, as opposed to NaH2PO4, which is sodium dihydrogen phosphate. But we wouldn't call Na3PO4 trisodium phosphate. The prefix is used only to distinguish between different possibilities, to alleviate ambiguity. Writing "sodium hydrogen phosphate" would be ambiguous. There is no ambiguity with "potassium chromate". Since chromate has an oxidation number of -2, then there can only be two potassium atoms.
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bcoz the cation (potassium) is just used to neutralise the negative charge on chromate anion.
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Just because ZnCl2 is called zinc chloride not zinc di chloride
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Because metals don't get a prefix, only non-metals with other non-metals.