The duopoly-the Republican and Democratic Parties-claim to not agree on much, but they certainly agree on one thing: Only Republicans and Democrats are permissible candidates for election to public office.
It’s not that the duopoly fears that third parties(or even radically different alternatives within their own parties) will win elections on anything like a regular basis. What they fear is that voters will notice that there ARE alternatives, even mild and reformist alternatives, to their continued and unquestioned rule. The status quo is in a precarious position: It can no longer afford for its subjects to think. Its survival requires an electorate composed entirely of coin-flipping robots.