A lot of it has to do with first-past-the-post voting (winner-take-all plurality voting), as well. The system's "spoiler effect" incentivizes people to vote not for their true preference but for the candidate most likely to beat the candidate they hate the most. Such a voting system inherently devolves into a two-party or one-party political system, and it greatly magnifies the success of primary election candidates who are well-funded and pushed by the media and party leaders as "front runners."
While first-past-the-post voting is perhaps the worst serious voting system ever conceived, not all countries with this system are as politically broken as the United States, because having an international empire magnifies the effect of media propaganda, arrogance, and the importance of "winning" and being the "strongest." Jingoistic nationalism has a direct impact on many of the policies the major parties (and media) push, and it sets the stage for the wider "team sport" mentality that idiom noted about party politics. The team sport mentality even affects primaries: As you may have noticed, the Republican leadership tried to coalesce the whole party around Romney for "party unity" many months before the GOP primary was even over.
New Zealand used to use first-past-the-post voting for their (your) House up until 1996, but now you guys use mixed-member proportional representation (as I'm sure you know

). Even when you had plurality voting, it probably wasn't as bad as it is here, since you don't live under the corrupting influence of imperial propaganda.
Plus, you know, we have Diebold and pals, and the 2008 and 2012 Republican primaries showed hugely suspicious statistical patterns linked to the central tabulators, and party leadership likes to strip delegates and sic cops on people supporting the "wrong" primary candidates at county/state conventions, etc.

Changing the voting system is a bit easier said than done, since we need to get into power - through the current broken system, no less - before we're in a position to do so.