It grew organically out of the "first past the post" system of electing officials. In the US, the candidate with the most votes in a district wins. This system is relatively unique in democracies, with most using proportional representation systems (voters vote for a party, and the seats are distributed on a pro-rata basis). Only the US, Great Britain, and probably Canada use the first-past-the-post system.

The end result of this is the tendency to have two party systems. I'm not sure exactly why this is, but pretty much everywhere a first-past-the-post system, there's a form of a two-party system. GB has more than two active parties now due to regional differences and the fact that Labor is pretty much disintegrating, but for awhile it was just the Conservatives and Labor (which supplanted the Liberals).