6) The American election system – winner-takes-all – guarantees that any third, fourth party etc has no chance of winning. To succeed in the national election, you simply need a majority of Electoral College votes. Only the Democrats and Republicans have any chance of gaining this. As such anyone who votes for an alternate party is effectively wasting their vote.
7) The American system allows both parties to adopt and shape their policies as and when they like. Therefore any decent policy taken on by a third party can be usurped by one of the big two – and that is acceptable in US politics. Therefore by adopting a third party’s policies, the ‘big two’ can move forward with policies that might be part of a third party’s strategy for success – and steal its clothes.
8) The media – a vital part of US politics – will only be interested in the Republicans and Democrats thus starving any other party of the oxygen of publicity. Likewise those who donate money will only be interested in likely winners – and that will only come from the Democrats or the Republicans.
If, like me, you ever wondered why America only has two parties? Or if given this election cycle we might get a more right wing and/or a more left wing party under the Trump/Sanders situation, this website explains some reasons why it might not happen.