Why, then, did the culture of bipartisanship disappear? Well, first, the two parties sorted themselves out ideologically, with white Southern conservatives joining the GOP and Northern liberals joining the Democratic Party. And second, the conservative movement took over the Republican Party. The old, moderate GOP establishment is long dead, having breathed its last gasps in George H.W. Bush's administration.And so the two parties don't work together now because there's no reason for them to do so.
This looks about right to me.