Your forgetting that back in the day of 'Main Street' conservatism traditional values regarding sexual morality were the established consensus. Over the past forty years the US (and Europe) has seen a dramatic change in how sexual behaviour is judged by society. It is natural that conservatives should become a lot more militant about such issues when that social consensus has crumbled.

With regard to economic issues I'd say it would make more sense to see the period from the mid fifties to the mid seventies as a temporary swing to the left by the Republican party. They opposed all government programs from Social Security to Medicare and Medicaid when they were proposed. So their views on that part of the political battlefield are also consistent with the past.

To sum up - not much has changed except a feeling by conservatives that their ideology is being undermined by a changing world, and it is the very essence of conservatism to become upset under those circumstances. Before you could have a de Maistre you needed the Revolution. The precondition for the rise of the Christian Right was the sexual revolution.