Because we can't find any Republican policies that have stood the test of time. Also, many of the stated platforms are disingenuous or contradictory. Especially, in both cases, Republican economic policy.

It was about trickle-down economics. This didn't work, and produced one of the largest redistributions of wealth in US history, conglomerating in the upper class. It was about reducing taxes and reducing government spending. But under Republicans, taxes are reduced, mostly on the upper class and businesses, but spending continues to increase. It was about reducing regulation, but that only ever ends up creating a crisis. (Do I really need a link for this one?)

The social policies I (vehemently) disagree with, but understand. But many "technical" policies, the economic ones in particular, have been shown to simply not work.