The parties themselves own more voter information in their datasets than CA ever dreamed of.
It's one of the reasons for why third parties can't break into the American system - it's hard to compete without access to a curated-over-decades dataset of likely voters, swing voters, potential volunteers, single-issue voters, etc, etc.
The US is a two party system[0] because it uses first past the post elections. First past the post electoral systems can't support more than two viable parties.
[0] Nominally. In practice, the commonality of ideas between elected Democrats and Republicans, combined with structural advantages that give the GOP a disproportionate hold on power, mean that the US is more of a 1.5 party system.
