Mostly American politics works like this:
some small number say 5% are at the extreme ends of the political spectrum (pick an issue, doesn’t really matter, will be about the same.) Of the remaining 90%, about a third are actually not really committed to either side. They swap back and forth on issues, and will support a candidate mostly based on a perception of the actual candidate, rather than needing complete assurance that there is agreement on every issue. The other sixty percent are party affiliated, and care more about keeping their party in power than any single issue.
Now, once someone is in office, the reality of our system is that parties select from their elected members for positions of power, such as committee assignments, appointed offices when they are in power in executive offices (Governor, or President). The majority party has more such power than the any other party, and aside from a few who caucus with another party, such positions are rather tightly controlled.
However, the US Senate has very peculiar rules that make even a significant majority control of the body less powerful than one might imagine. Because of this, The Democratic party is unlikely to fail to work hard against some progressive left leaning smaller party, since the existence of their party is an impediment to really moving their agenda. Many state legislatures have analogous power structures. Since there are no “votes of confidence” during the term of office, there can be no realignment of minority parties into a “coalition” government such as parliamentarian systems regularly have.
This causes some really odd political happenings. The Bush administration failed to marshal their extreme wing during the last half of his second term, because they had a lot of difficulty avoiding losing the uncommitted center. At the same time, a very large fraction of that center expressed their dissatisfaction by actually turning out and working for the opposition party during the off year elections, and without that moderate end of the Republican party speaking out forcefully, the more strongly conservative wing of the party gained a lot of control over the party itself.
What didn’t happen was a building of a base of support for the Republicans among the uncommitted center. The center isn’t a fertile area for the growth of new parties, and the right wing believed they were gaining power, so they rode the tide, believing that the silence of the majority actually was an opinion. So, there was very nearly a strong super-majority elected to the Senate. For a refreshing change, the left actually decided to all walk along together as if they were an organized political force.
So, mostly, the fight is over the middle third of the engaged electorate. They have no platform around which to form a party. If you have enough conservatives or liberals to take an office, or offices, you have enough to take over the Republican or Democratic party itself, so only the far five percent are likely to form a new party. (Single issue parties almost always actually support one party of the two largest, with aberrations now and then, like Perot, Nader, etc.) The reason is that you cannot get anyone elected with five percent of the vote, and if you get someone elected, they get to make speeches, and submit bills, but never see such bills reach the floor of the legislature. To do that, you need majorities in committees.
So, the reason is that the Federal system was actually designed to facilitate two parties, and deny power to anyone not involved in one or the other.
Tris