system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are there only two main political parties in the United States of America?

Right or left? Other countries seem to have more than two. If America is so progressive why no Variety?
gpt
Our winner-take-all voting system makes it *mathematically optimal* to have two parties.  If a strong 3rd party were to occur, its stances (and its voters) would rapidly be taken over by one of the other major parties.  

Also, I've never heard of the US being described as progressive, at least not since the early 20th century!