ELI5: Why do only the two biggest parties in the US a chance to win an election?

It's because whomever gets the most votes, gets the office they ran for.

Say there is an election for a state governor this year. The candidate that gets the most votes is the one that gets the position. Let's say there are four candidates running, each representing one the following four parties: Ones, Twos, Alphas, & Betas. The population is mostly evenly distributed among the parties so that each have 25% of the vote. Of this election, whomever gets the most votes wins, so they focus on getting their people out to vote. Now, Ones and Twos have much more in common with each other than the rest, so they decide that it's better if they team up because while they may not be perfectly happy if they win, they'll be more happy than if either Alphas or Betas win. They create a joint party called Numbers. The voting population is now distributed so that Numbers have 50% of the vote, while Alphas and Betas have 25% each. The Numbers party will now easily win all elections. Alphas and Betas see this, and they decide to join their parties too, coming out with the Alphabets party. What was once four parties became three then two.

This is an effect that results from having a winner-takes-all (i.e. first-past-the-post) type of election. In places where people vote for parties of their legislature, and the legislature then elects their executive leader, you get more parties. People can vote for their favorite party that represents them best. Those get into legislative office. Then, those parties can team up to promote their favorite candidate for leadership. In this manner, everyone has a more detailed representation, so the legislature represents the population more correctly. The downside is that the leader is not directly voted on by the population, but the politicians.

I may be a little incorrect on how the details work in the second paragraph, but that's the general idea. If you want to see a video that explains the first paragraph using animals as parties, check this CGP Grey video out: https://www.youtube.com/watch?v=s7tWHJfhiyo

Thanks!
- You got it, dude 👶👍

Dunno why you're being down voted, it's a legitimate question. Our electoral system is the reason. Only the highest vote percentage gets into government. So third party candidates can only be spoilers in most elections. To prevent that, the two major parties scoop up any serious third party candidates, or do their best to discredit them.

There is a recent change in a few states to ranked choice voting, that aside from selecting more centrist candidates, also gives legitimate third party candidates a place. Still not enough really get them off the ground, but very much a way to make voices heard without sacrificing a seat in government.

I think it's important to note that this system is quite different from OP's country, Germany. And it's also important for Americans to understand these differences.

In the US, you vote for single candidates for Congress to represent your district. In the lower chamber ("House") the districts are pretty small, and cover about 700,000 people*. In the upper chamber ("Senate") there are two Senators that both cover an entire state. The single candidate with the most votes wins the seat.

In Germany, part of the parliament is elected in a similar way to the US, but the other part is very different. You vote for a party, and the seats in parliament are distributed proportionally to the percentage vote across the whole nation.

The German system has provisions to ensure that parliament is roughly proportional to the total popular vote for parties. The US has no such system, and without it, game theory demands that only two parties can be competitive.
Dunno why you’re being down voted

Ah, I have stopped wondering about that ages ago. Must be a Lemmy thing.

Our electoral system is the reason. Only the highest vote percentage gets into government.

Thank you.

Still not enough really get them off the ground, but very much a way to make voices heard without sacrificing a seat in government.

That's great! :-)

first past the post elections encourage a 2 party system.
it's not a parliamentary system, so a legislature hostile to the executive is more probable with a multi party system, so the 2 parties are already coalitions.
the winners of elections get to set the rules for future elections so both parties work to make third parties more difficult. This factors into federal funding, qualifying for ballots, and debate appearances.

Because nobody has a chance to win except Democrats or Republicans, voting for anyone else is "wasting your vote." Which is why nobody has a chance to win but Democrats or Republicans. It's a self-fulfilling prophesy.

mixture of advertisement/visibility, and the perpetated idea that because a 3rd party is unlikely to ever win, some people vote for the less worse option of the two due to not having ranked choice voting

https://en.m.wikipedia.org/wiki/Duverger's_law

The two ruling parties have a very long history in this country, are backed by the billionaire class and therefore have unlimited funding, and they use their power to suppress third parties nationwide so voters only ever have two choices.

Yes there are other choices, but it's exceedingly unlikely and rare for a Non-Democrat and Non-Republican to get elected anywhere other than locally, largely due to the monied interests backing the two ruling parties.

they use their power to suppress third parties nationwide so voters only ever have two choices.

How do they do that?
- There are several ways. Some states have made it harder for candidates to get on the ballot, which is one method of suppressing third parties. Sometimes they sue and challenge a third party's ballot status, as was done to Jill Stein several times this year. In some states, they don't really have to, as they can use their unlimited funds to ensure their names are the only ones heard in the media.

People make their party affiliation a big part of their identity. It's hard for anything not already established to compete with that.