Basically because we were early adopters to modern republic systems. We tried something new because parliament was a bit too kingy for our tastes. But due to its simplicity it became really easy for two parties to wipe the floor with everyone else. And basically the only times they’ve changed was at the start and again shortly before our civil war. Neither party has ever had good reason to change the system, which would require massive agreement to change our constitution. So nobody does.
For example, politically I’m a syndicalist, but the democrats are pro union, pro environment, pro woman, and pro lgbt, all of which with a big asterisk but still I consistently vote for them because the greens didn’t win with Nader so they’re definitely going to lose now. So I dutifully vote Democrat because the only other party that has a chance is the republicans and they hate me and everything I believe in.
If we could do it again we’d do it better but in our defense we didn’t really have anyone to model off of