In short, a "Winner Takes All" (aka "First Past the Post") voting system, the kind the US and dozens of other countries use, where each person gets one vote and the candidate with the most votes wins, is absolutely terrible and inherently favors a two-party system among having many other problems.
This can be seen mathematically with game theory and probability, and is also explained in political science by Duverger's Law. But if you want a demonstrative video, there's an old but good one by CGP Grey:
Essentially, given a choice between many candidates, people have more complicated preferences than just a single person they want to win. There's the guy you want to win, the 2nd guy you want to win if the first fails, the 3rd, and so on, until you get to the people you either don't care about or hate the most.
But in a winner takes all system, you don't end up simply voting for the person you want to win. Strategy dictates to instead vote for the person you dislike the least out of the few that have the greatest chance of winning. Voting for someone whom you know won't win is a waste of a vote as it won't affect who wins.
And it turns out, to no surprise, any kind of preferential or ranked voting system blows winner takes all out of the park. In such a system, you would simply rank the candidates in the order you want them to win from first to last, no strategy needed. Those left unranked would be interpretted as being tied for last.
If this gets any interest, I can make more posts going into more detail.
