Why these two choices

Why aren’t libertarianism, populism or other political leanings more popular in American political-culture than the simple left-right dynamic? This TNR article on the religious non-right is pretty interesting in that regard.

The two major political positions, of government control in social matters with economic freedom versus freedom on social matters with government control in the economy, are coherent not just philosophically but on a very visceral level that’s easy for many people to intuit themselves. “Individual responsibility” means allowing individualism in the economy and encouraging responsibility via the government, according to TNR:

In researching their book, Divided by Faith: Evangelical Religion and the Problem of Race in America, Michael Emerson and Christian Smith found that evangelicals are more inclined than nonevangelicals to blame an individual's failure to thrive on personal shortcomings--say, a lack of ambition or character--rather than on any systemic disadvantages. By extension, write the authors, "Because systems and programs are viewed as obviating personal responsibility and not changing the hearts of individuals, they are ultimately destructive." Thus, "Welfare is seen as both terribly misguided and sinful, running counter to most things American and, in their understanding, most things Christian. It is far better, according to this [representative interviewee], to 'give them the basics of God and teach them about Jesus. That's going to bring them a whole lot more out of poverty than it is to give them a welfare check.'" Or, as Bush is so fond of asserting, the best way to tackle social problems is by changing "one heart and one soul at a time."

And it’s not just philosophically coherent, but there’s a whole class of people who naturally will feel both sides of this position (the economic and the social). And similarly for the left, though in a vice-versa way.

It's hard therefore for the left to convince the religious right of even some of their platform, or for there to even be a strong religious movement outside right or left.

So while the “bichromatic spectrum” is roundly mocked in America for being silly to presume they are the only possible political approaches, there are good reasons (besides oligopoly) for those two positions to be the dominant ones.