What's the Political Science Phenomenon of Two Candidate Competition?

The two partie system that we have in america is thought to be a strategic outgrowth of the predominant use of plurality winner elections. The political science phenomenon that i observes is two party competition. But what's obvious in the primaries, and especially this year, is that this so called law has no purchase in a primary election. It is not causing the interest groups or factionsn each party to coalesce around two alternatives.