The US 2 party system exists largely because of the structure of our government. The Western European parlimentary and prime minister system is more condusive to the formation and existence of minority parties because they can very often exert much greater influence on the major parties than their popular support would lead you to believe.
Take for example the US system. Let's assume that between 40 and 45% of the population is either going to vote for the Democrats or Republicans. This leaves about between 10% and 20% of the population as independents. If the people truly voted independent and didn't support one of the major parties, their votes would be at best meaningless and at worst counterproductive to their goal. Everyone knows that a vote for Nader is effectively a vote for Bush, the same way that a vote for Perot was a vote for Clinton.
In a parlimentary system, the leader has to be chose by an absolute majority of the
MP's therefore, unless one party can gain a clear majority they must work with the other minority parties to establish a government. This is not the case in the US system.
I believe that the need to form coalition governments serves as a moderating influence to change as any very dramatic change in course could result in the splitting of the coalition and the fall of the government.