system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come the US just have two political Parties?

In Europe, they have like 100s of them.
gpt
So let's say there are 10 people who want to get elected.  It's a winner take all election, whoever gets the most votes is going to win and gets to make all the decisions.

Well, when you go through the votes, its' going to be clear some of those 10 people are more favored than others.  Bob is not very favored, they estimate only about 5% of people are likely to vote for Bob.

If Bob just stays by himself and loses, he ges nothing.  Bob mainly wants to make it a rule that everyone gets to eat pizza on friday.  He has other stuff he'd like, but that's the important one.

So Bob, knowing he can't win and wanting to make sure everyone gets pizza on friday goes to Tom.  Tom is a front runner in the race and doesn't particularly care what people eat for dinner on Friday.  Bob tells Tom if he promises to make the pizza on fridays rule, he'll drop out of the race and tell all the people who would have voted for him to vote for Tom instead.

Now, this is great for Tom, remember he doesn't care who eats what on Fridays, but those votes will help him get elected, so why not?  He agrees, and now Tom and Bob are in a political party.

Bob isn't the only one to figure this out.  Pretty soon everyone has asked for something from the two most likely to win, because there is no reason for them to continue running on their own when they know they won't win.  It makes more sense to ally yourself with a more powerful candidate more likely to win in order to gain some influence even if you don't agree with all their ideas, instead of running on your own, losing, and getting nothing.

That's how the American system works.

Now the European systems are all different in different countries but basically instead of a winner take all, it's weighed to how popular you are.

Instead of electing one leader, everyone votes to decide how much say each leader gets.  So Tom wins the election with 40% of the votes. The doesn't seem like much, but remember there are 10 people splitting up the votes, so the average would be only 10%. 

Now, Bob brings up the pizza on Fridays rule for a vote, and gets 5% of the vote.  Now he can make deals with Tom and others to make sure his rule passes, but he doesn't need to join Tom.  He can stay his own party and influence votes on many different issues, not just pizza on friday.  His 5% of the vote might be the difference in a contested issue, so he can make deals to exchange his support for support on the stuff he wants.  In this case it makes sense for him to stay seperate instead of joining Tom.  

By not being winner take all and leaving some consolidation for Bob, he's more likely to stick it out alone.