Here's something to ponder. A lot of democracies have preferential voting. You vote in order of candidates and then counting votes is done in rounds where the lowest voted candidate is eliminated and those votes go to the next preference. This works well in avoiding a 2 party system since you can vote for third parties but still have your vote counted no matter which way the count lands in the end (eg. if it does come down to the 2 largest parties you haven't wasted a vote on a third party).

When there's no preferential voting system and therefore only two real parties in the political race it's easier to ensure you get the outcome you want either way. PACs don't really need to influence the election directly as much as ensure they have influence on politicians in the only two parties that have any power.