There have been many. Early USA was a free for all. Whigs die out, republican/democrat duopoly follows, with brief challenges by the Bull Moose, Perot, Forbes and such.
Why not ? just money, i suspect. But as we see with trump election, the duopoly is breaking. Both parties are losing credibility by the minute.
Greens ? Libertarians ? or perhaps (gasp!) socialists and communists ?
sidd