The easiest way to do voter fraud and get away with it, works best in places where one party runs a state as its own slush fund. As an example, about 60% of the eligible voters voted, nationally, in the last presidential election. That means 40% of the population did not vote for one reason or another.
What would happen if political insiders, buffered by a one party state, voted for those who never vote? It would not be hard for local officials to compare census data with voter registration data and voter habits, to see who can vote but who never votes. On paper such votes will look legitimate, while the person, who never votes, but who has officials vote for them, will never know.
If outsiders were to look at the voter data, they would only see eligible voters. Investigative groups would need to go door to door and talk to each voter, one by one, and ask if they voted. This could explain why certain states do not wish to share records even if kosher on paper.
For example, the vote turn out in California for the 2016 presidential election was 75% of eligible voters, while the national average was about 60%. If we multiple the 19.5 million eligible voters of California, by the 15% difference, we get 2.9 million votes.