The short answer to your question is "Duverger's Law".
The U.S. government's organization and method of choosing candidates to fill seats greatly discourages the co-existence of more than two major parties, as any similar government organization is inclined to do. The U.S. government has single member districts or seats rather than proportionally represented multiple-member seats, and these seats are filled using the "vote-for-only-one-candidate" plurality voting method. Any government that possesses these two traits is very likely to be comprised primarily of only two major parties. This observation about governments was originally studied by a French sociologist named Maurice Duverger and is consequently named Duverger's Law.
From Wikipedia site:
"A two-party system often develops from the single-member district plurality voting system (SMDP). In an SMDP system, voters have a single vote which they can cast for a single candidate in their district, in which only one legislative seat is available. The winner of the seat is determined by the candidate with the most votes. This means that the SMDP system has several qualities that can serve to discourage the development of third parties and reward the two major parties."
If Duverger's Law is indeed correct, then other hurdles typically blamed for stalling third parties such as financing limitations, corporate influence, electoral college method of electing a president, voter apathy or ignorance, poor media coverage, ballot access laws, etc, are not primarily responsible for locking the U.S. electorate into two-party dominance.
The way I envision it, the single member district plurality voting system within the U.S. erects an essentially impenetrable wall for third parties. On the other hand, third party burdens like ballot access laws are hurdles that could be overcome with some effort once the worst of all voting methods, plurality voting, is replaced with something much better like range voting, approval voting or a Condorcet method. In the event that such a change is not adequate to bring about a multiple-party government (creating multiple-member districts also may be necessary), at least it is a step in the right direction, and many other benefits would be realized by getting a much better voting method than plurality.
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