It's a duopoly/monopoly. The elections are rigged, in the political process, media, and electronic voting machines.
What did you expect? It is a multi-trillion dollar "racket." You think these criminal cockroach mafia are going to leave things to chance or the will of the unwashed masses?
Elections aren't rigged, fact of the matter is that the winner-take-all American electoral system is pre-disposed to favor a two party system. If it was proportional representation, as it is in Europe, third parties would be more relevant. But the winner-takes-all elections preclude third parties from being serious contenders.
Oh, OK, the elections are "rigged". Yet every so often you get sea changes like 1994, 2006, and 2010, when a party with a large majority in the House loses it. In 2010, for example, Republicans had a net gain of 63 seats and went from a minority to a large majority. Was it "rigged" that way to fool us into thinking the voters have a say?
As usual, you go way over the top with your conspiracy kookist nonsense and ruin even more of your credibility, if you had any left.