What Brings About the 2-Party System?

A two-party system is the outcome of America's founding political structures
A two-party system is the outcome of America's founding political structures and modern obstacles to 3rd parties
U.S. electoral system is winner-take-all: a fraction of the votes will not translate to an equal fraction of power, discouraging 3rd parties
Only one third party has gained the presidency in American history, after which it became one of the two-party system: Republican Party
In other systems, fractions of votes lead to fractions of representation, thus increasing the rewards for third parties
Both Congressional and Presidential elections are winner-take-all
The German political system has party-proportional voting and has many parties, a likely outcome of that system
Congressional districts are often single-member districts, meaning only one candidate with a plurality will be elected
Presidential elections are through the Electoral College, which has only a single winner
Modern obstacles are opposing the growth of third parties
Third parties receive fewer federal subsidies for the presidential election and are forced to raise more of their own money
"Public Funding of Presidential Elections." Federal Election Committee Website.
The Commission on Presidential Debates has standards that are not feasible for third-party candidates to meet
'Third Party Candidates' Mission Impossible.' BU Today.

'Third Party Candidates' Mission Impossible.' BU Today.