Trying something new: America AMA! 

Ask me a question why something is the way it is in the US and I'll try to give you the best answer.

Qualifications: I've been an American my entire life. Other Americans should feel free to chime in.

Duopoly dynamics post-Civil War, a lot of small changes (ballot access restrictions, public funding, access to important committees in Congress) and then historical norms (winner take all even in a plurality contest). 

Wasn't always the case:

Duopoly dynamics post-Civil War, a lot of small changes (ballot access restrictions, public funding, access to important committees in Congress) and then historical norms (winner take all even in a plurality contest). 

Wasn't always the case:

https://en.wikipedia.org/wiki/1860_United_States_presidential_election