Our non-parliamentary, first-past-the-post system causes that. We’ve always been a two-party system. The parties that are those two parties have swapped (no more Federalists or Whigs) and realigned (Andrew Jackson would be a bit surprised by the current Democratic House Caucus, and Abraham Lincoln would probably be a bit surprised, as well), but outside of the independents who sometimes win seats in New England (but are pretty aligned with the Democrats), it’s effectively a two-party system.

Conclusion I’ve come to while comparing systems with my German colleagues: in Germany, the coalition negotiations happen after the election; in the US, they happen before the general election, via the primaries.

I’m curious to see if Louisiana and California “jungle primaries” might begin to break this up a bit after a few more cycles.