Because when the right is in power they fall in line, and when the left is in power the fall in love.

Explanation, each left leaning congress critter and senate sloth has something they want to champion, something they usually got elected to do or fix… And they are wildly inconsistent within the party on how to do that. With the right being obstructionist cockwombles most of the time it then appear from the outside that left is incapable of getting things done.