Another severe element is that third parties are generally bereft of talent as anyone that has the skills for politics or government will choose the path that gives them the greatest chance of achieving either elected office or some position in administrative government.

Combined with this, the institutions that actually perform the business of governing will also be tailored toward serving the interests of the parties. Though - such as in the case of the Military, CIA or FBI, often it seems the tail is wagging the dog and the two-party system serves the institutions interests. Their interests are not served by third party challenges.

Also, naturally, the parties are fueled by funding and this leads to a great deal of private institutions and industries - like banking and investing or oil companies - that are outside government but have a much greater influence on the form and function of the government. They also like having simply two parties to play off of each other.

As a result, third parties often have leaders that are less politically-savvy, less organized and administratively competent and far more poor than the major parties. Then, if any third party does get traction, it usually is either formed by some wing or powerful members of one of the major parties and is not legitimately a third party or it will eventually be merged into one of the major parties it most closely resembles on the arbitrary left-right scale.

Essentially, United States government seems to fall prey to the same problems that its corporately dominated industries. To be completely honest, many of the major third parties are guilty of the sort of naiveté, incompetence and unattractive-to-voters claims that the major parties accuse them of and are basically a destructive rather than creative force in American politics. However, much of that is due to the major parties rigging the system so that will be the case.