The biggest source is the rise of polarization. With a more liberal Democratic Party and a more conservative Republican Party, there are higher and higher levels of partisan team play in Congress. It reduces the incentive to accept a compromise. There’s also increased electoral competition. When was the last time we had a landslide? When the “big enchilada” [all three elected branches] is in reach, why not hold off until your party controls the White House, House, and Senate, and then legislate?