Even then, you only have a snapshot, because the states typically don't keep historical data. What this means is that your dataset won't be as good as someone who's been collecting this data for years, and thereby knows things you won't like where someone used to live, how often they voted there, who recently dropped off the registered voter rolls, etc.

In this case, even this data wouldn't be enough, because the Sanders team had made likely hundreds of thousands of contacts with voters, and recorded what issues they cared about and who they planned to vote for. This data, which they personally collected, is now inaccessible to them.

edit: expounded