I think SharpFang has a very good point about the corruption that ensues when big corporations and rich people get too much power. Another factor that counts against the USA is just that it is so big: over 300 million people in one of the largest countries in the world. That means there is hardly anything in common between people's interests in, say, Alabama, Alaska, and Montana. That, together with the strong "business" slant of the national culture, leads to Congress being less a forum for reasoning and exchange of facts and opinions than a mere commodities exchange. Senator X tells Senator Y, "If you vote for my bill I'll make sure your constituency gets a nice juicy government contract". And so it goes.