Because America has a first past the post election system, which will always result in two dominant parties. See https://m.youtube.com/watch?v=s7tWHJfhiyo for an explanation.
The US is one of the most Capitalistic and Imperialistic countries on the planet, and as such the parties available are the ones that uphold these positions. It's a positive feedback loop with power.
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The structure of the Constitution favors conservative movements because it's undemocratic and designed to resist change.
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Because too many voters only pay attention every four years and wonder why there is no bespoke candidate for them.
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There are plenty of such parties. They are just not electorally successful on a national scale. They may be moderately influential on a state level through the use of fusion voting. Fusion voting is where multiple parties can stand the same candidate in an election.
Most places in the United States use a "first-past-the-post" system. In this system, voters select one candidate and the candidate with the most votes wins. This system sounds fair on the surface but in reality, game theory dictates that the only stable configuration of political parties in such a system is a two-party system. In any other configuration, the individual actors will always find it more advantageous to join one of the two parties. The reason for this also explains why it's hard for smaller parties to win under a first-past-the-post system.
Suppose there are two existing political parties: Party A and Party B. Voters prefer Party A by a margin of 55-45, so Party A wins reliably in elections. Suppose we replay the same elections but with three parties. Party C holds similar views to Party A but is more extreme while Party A is more centrist. If everyone votes for their favourite candidate, then we will probably end up with a vote distribution where Party A wins 40% of the vote, Party B wins 45% of the vote, and Party C wins 15% of the vote. What has essentially happened here is that Party C siphoned votes away from Party A, causing Party B to win despite the fact that voters' preferences haven't changed. Voters know this and so even those who like the Party C candidate the most will vote for the Party A candidate (who shares at least some of their views) in order to stop Party B from winning.
This is why progressives forming their own political party is a losing idea in the United States. It will split the left-wing vote and hand elections to the Republican Party. Instead, what they do is compete in the Democratic Party's primary elections. In the US, who a party chooses to stand in a particular election is determined by means of a primary election. However, progressives often struggle to win intra-party primary elections because most members of the Democratic Party are moderate. The distribution of political leanings is shaped like a bell curve, and thus progressives like Bernie Sanders are simply outnumbered by moderates like Joe Biden. Moderates often have the numbers to sideline progressives in primary elections, and thus it is much more difficult for progressives to get elected since they need to run under the Democratic Party banner to stand any chance of winning.
Same reason there's no fascist party: the two main parties contain a broader range of the political spectrum than in most countries.
From there the question is does the moderate or radical wing of the party gain more influence. The far-right has won the Republican party years ago while progressives still haven't gained that much ground in the Democratic party.
What would constitute a political party virtually anywhere political parties are relevant is a political faction or caucus within one of the two establishment parties in the American system.
Progressives are generally a caucus within the Democratic Party. The Democratic Party is predominately and increasingly a centre-right party and has consistently thrown its political weight behind incumbent conservatives against its progressive caucus.
These are the major components of there not being an electorally relevant American Progressive Party.
the cold war, probably
The last time there was the government made it illegal https://en.wikipedia.org/wiki/Communist_Control_Act_of_1954
Because our government has the best propagandists in the world and they are aimed at us and it's working. Also, intelligence agencies sabotage efforts in their infancy.
The only test that exactly no one will ever pass is a progressive purity test.
There's always something objectionable and it gives them the perfect excuse to do nothing instead of something that's not perfect.
That's not exactly good for making a party, let alone a viable one in a first past the post system.
You need radical politics - not "progressive" ones. "Progressives" are far too easy to buy - or intimidate.
Because there's too many old voters.
Because money
One of the problems is that any party has to register on every fucking state in order to be recognized in the whole country. So you have parties that are stuck in a single state, because they can't get momentum/people outside in order to expand/exist.
