So why then does the US have two meta-coalitions instead of a multiplicity of little parties? I can eliminate one popular misconception right off the bat: There are no formal provisions for two parties in the US political system. (At least, none to speak of.)
The proof is simple: There are more than two parties in the US, something I daresay even most residents are only vaguely aware of. There's Reform, Green, Libertarian, and a handful of others. Clearly they are not banned from existing.
So, why then are they always marginal? Why does it take an act of Perot to get them to even make a splash? Because of the nature of the meta-coalitions.
Each meta-coalition is always tuning its collection of special interests. When a third party gets strong enough, the meta-coalitions take note, and basically co-opt the reasons for the third party being strong, in the process co-opting the voters, too.
But it isn't purely a crass play for votes; yes, there is some of that, but the party must walk the walk at least enough to keep people happy or they will lose the voters, so such co-option does have a real effect on the policies of the meta-coalition.
You can tell which meta-coalition has co-opted which third party issues by who the third parties are accused of stealing votes from. The Green party "steals" votes from the Democrats, because the Democrats took up the environment issues. The Libertarians "steal" votes from the Republicans, because the Republicans took up small government as part of its "Contract With America" reformation. It isn't explicitly in the contract, but it is implied strongly by the fiscal resposibility clauses and how they were intepreted at the time, if I recall correctly. (As a sidenote, it is interesting nearly ten years later to look at how well they did, and how far the Republicans have wandered in that time.)
But such things are not permanent, nor are they always equal. Right now, both parties seem to stand for large government, and at least in some of the circles I frequent, the Libertarian party is being talked about much more than in 2000 as a result, because the Republicans are not keeping up their end of the vote bargain for the Libertarian vote. They still talk the talk somewhat but they are not walking the walk.
While I don't think the Democrats have stopped trying to be environmental, a lot of the Green voters are probably expressing their discontent with the Democrats inability to do anything meaningful from their point of view on the environment, because of the current Republican dominance.
Despite party-line rhetoric to the contrary, there is nothing except inertia preventing the Democrats and Republicans trading their big industry and environmental interests tommorow, and life would basically go on as usual. Lesser interests are in fact often traded, though usually over a period of a decade. One party loses interest, and somewhat later the other percieves an opportunity to gain votes. Or perhaps a party reforms itself and reaches out to some interest group and succeeds in wooing them away from the other meta-coalition; that can happen more quickly.
The two "party" system results from emergent properties of the United States Goverment system, which is to say, it is completely informal as defined in my introduction to this series.
(This is #2 in the government myth series.)