There are a few reasons: "Partisanship has been attributed to a number of causes, including the stratifying wealth distribution of Americans [2]; boundary redi

There are a few reasons: "Partisanship has been attributed to a number of causes, including the stratifying wealth distribution of Americans [2]; boundary redistricting [3]; activist activity at primary elections [4]; changes in Congressional procedural rules [5]; political realignment in the American South [6]; the shift from electing moderate members to electing partisan members [7] movement by existing members towards ideological poles [8]; and an increasing political, pervasive media [9]." This paper also attempted to quantify the polarization: "We define a network of over 5 million pairs of representatives, and compare the mutual agreement rates on legislative decisions between two distinct types of pairs: those from the same party and those formed of members from different parties. We find that despite short-term fluctuations, partisanship or non-cooperation in the U.S. Congress has been increasing exponentially for over 60 years with no sign of abating or reversing. Yet, a group of representatives continue to cooperate across party lines despite growing partisanship." http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0123507 http://journals.plos.org/plosone/article?id=10.1371/journal....