As has been said a billion times, the issue isn't that the two parties are too popular (obviously) or even too rich and powerful, it's that our election rules make it extremely difficult for more than two parties to compete in a stable system. The congressional elections with high-population FPTP single-member districts in particular are so extremely weighted against third parties that they may as well not exist, and as most state governments mirror the congressional system, they hardly have a chance there either. The whole American political system is generally undemocratic and unresponsive whether third parties are desired or not, but mysteriously this issue never comes up in the national media aside from the odd reporting on gerrymandering.
The argument that the parties are internally varied and more like a coalition falls flat in my opinion, since even if this were true in the past, the development of 20th century telecommunications and mass media has so extensively homogenized american culture and politics that regional differences are weaker than ever, and where they do manifest in politics will largely only do so within the two-party framework (i.e. little actual deviation from the mainstream, just varying in intensity and in particular around strong single-issues).