Two-party system: Black & white? Or red & blue?

In American politics, it is common knowledge that we have two main political parties: Democrats and Republicans. It's been this way for decades, but why are there only two major parties?

1. There really is a lot of consensus on broad issues in the U.S.

• Capitalism and the idea of a free market
  
• Freedom of the press
  
• National security (we should have it)

2. History

• There were two factions when the Constitution was ratified, and it's just never been done differently

3. Our electoral system

• We have single-member districts where the winner takes all
  
• This is different than proportional representation, where seats are awarded based on voting percentage
  
• First-past-the-post voting system: The winner is the one with the most votes, even if it's not necessarily the majority

This kid is brilliant.