The only population registry we have is the census every ten years. Other than that tere isn't any sort of central population registry, although I suppose in a nightmare scenario if there were a couple strategic data breaches hackers and/or other interested parties could probably create a de facto one. As to why voting registry is separate from other stuff, I'm not exactly sure, but suspect it has to do with fights between the two major political parties over how individuals identify themselves when voting: Some want a specific separate voter ID while others say the one you already have is just fine..