Cant eartag someone or brand them or tattoo them, cant impose it on a phone which might be thrown out or swapped with somebody else. Stick in the blood of people permanently and then ban transfusions? That really doesn't work either. Not that transfusions are super common, but maybe common enough for that scheme to be nonviable.

DNA is the single most unique and immutable attribute people have. Maybe the PCR tests were the real villain all along and was always intended to harvest this info primarily.