The US system and even its language is heavily tailored to two parties (state-run primaries, the term bi-partisan). This is barren ground for new ideas and thus the parties serve the public poorly. Scared of upsetting crucial swing voters they forsake grand visions. Yet they also polarize for its own sake, hunting for wedge issues to divide opposition support. From the mix of bland and divisive policies voters must choose the lesser of two evils. Voters need choices that allow for a wider spectrum of ideas.
Coalition governments, sometimes perceived as weak, can in fact yield consensus. How can we add more parties? Britain is also dominated by see-saw of two main parties. While it is recognized there that real change requires different approaches (proportional representation, perhaps) there is little incentive for either main party to vote to dilute their own power. So they don't. In the US there's an added hurdle: the sheer cost of politics limits the chances of any new parties.