Matthew Yglesias reminds us of the racial apartheid that made the bipartisanship of mid-20th century America possible:

[I]t really is remarkable that for all the bellyaching about the decline of bipartisan behavior in DC there’s very little attention paid to the fact that there are actual reasons this has happened beyond Newt Gingrich being a meany and bloggers being too shrill. The Jim Crow South gave rise to an odd structure of American political institutions whereby both of the parties contained substantial ideological diversity. This had the benefit of setting the stage for a wide array of cross-cutting alliances. It came, however, at the cost of consigning a substantial portion of the population to life under a brutal system of apartheid ruthlessly upheld through systematic violence.

After that system collapsed, there was a two decade or so period during which the voters and parties were re-aligning themselves during which we had cross-cutting alliances but no apartheid. And now the aligning process is done, so we have two parties where essentially all Democrats are to the left of essentially all Republicans and so you have relatively few genuinely bipartisan coalitions.

Indeed. Few people recognize that the previous bipartisan era was an aberration, not the historical norm. Consider, for instance, this plot of estimated party polarization in Congress:

When Democrats are disaggregated by region, the picture is even more clear (these graphs are for the House; those for the Senate are similar):

In short, the rise in bipartisanship was driven by Southern Democrats. Now that they are an endangered species, we’ve returned to the historical norm of sharp partisan conflict.

(Note: For the definitive analysis of how the race/civil rights dimension became aligned with partisanship, see the classic Issue Evolution by Carmines and Stimson.)