system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people from the US never consider voting for a party other than the Democrats or Republicans?

I hear comments from people that lead me to believe that their choice of party essentially boils down to picking the lesser of two evils and I cannot understand why they wouldn't simply support a party who actually represents their views.  
I understand that those parties are very large and powerful but that doesn't mean they're the only options.   I also understand the perspective that there's no point in voting for a third party because "they'd never get in anyway because everyone else votes for the big ones" but doesn't this miss the point entirely?  If enough people disagree with the big parties and vote for another one then the other one can win.  
It seems as though people have a fundamental misunderstanding about how democracy works.
gpt
Do you know how single member district plurality voting systems work? It's the voting system used for national elections in the United States.  People vote for a single person in each voting district, and votes are tallied at the district level, with the person with more votes than any other winning the election in each district. There is a principle called Duverger's law, that says that voting systems like this tend to result in 2 party systems on their own, regardless of intentions.  

Smaller parties tend to fuse together or die because they only receive any recognition or power if they receive a plurality of votes.  Getting 15% of the vote is useless, so banding together even though you might disagree on some things makes sense.  A party that gets 3% of the votes will not have their voices heard at all if they poll 3% in every district.

Having multiple moderate candidates and one or two radical candidates tends to work against the moderates and for the radicals because it is easier for the radical candidates to find a base of voters.  Moderate candidates simply distract the voters.

Third parties really only have a chance at success if the voting system changes somewhat (proportional representation being a popular option) or if they have the ability to take the place of an older party, as has happened a few times in US history.