Earlier quoted context omitted.
Voter registration is considered public information in many states. Some states even provide the entire database on their website to download. However, voter registration does not include who a person voted for in an election. You are free to augment the database with your own data, of course.
I don't even understand why party affiliation is tracked by the state. What's that good for other than entrenching the two party state? Parties should have their own member lists.
This, and ‘because that’s how we’ve always done it’ are probably the main reasons party affiliation is part of voter registration.