Right wingers think they are too nice and so the left always wins. Left wingers think they are too nice and so the right always wins. In part this is due to our belief that the ingroup is pure and the outgroup is corrupt but I think there is a grain of truth to both perspectives: the right has the edge in economic battles while the left dominates the culture wars.
This has something to do with Conservatives insisting that their economic opponents are a bunch of communists and progressives maintaining that their cultural opponents are a load of bigots. Yet I do not think this tops the reasons for these trends. One that is overlooked is our bias towards prioritising first order consequences. People tend to be averse to leftist economic ambitions because they entail tax raises and we don’t want to lose money from our pockets. People often care less about leftist social ambitions because, say, abortion or gay marriage don’t have any immediate effects on our lives. This is even more true among the elites, who often have the smarts to think about second and third order consequences but are better shielded from their effects.