One of the things most people don’t understand about the current state of American politics is that most legislators don’t fear general elections. They fear primaries.
The recent death of Scalia demonstrates this. Why did the Republicans immediately say they wouldn’t approve any justice instead of using their leverage to get a more conservative candidate. A tweeter explains:
Why is this the case now? A perfect storm of urban/rural demographics, the Big Sort, gerrymandering, and the decay of party power. Money makes a difference too, in that external money can now match what the party used to provide.
This is true on the Democratic side as well, though a little less so, as Democrats are generally a bit less secure in the general election due to urban/rural dynamics and gerrymandering.