Because they feel that in today's political climate that they would have easily gotten a pass. And had it been another corrupt shitstain as president, it's likely he would have gone through anyways.
But because we're coming up on mid terms and protection of Trump is riding on retaining their majority, they can't risk it on people like Kavanaugh. McConnell and Grassley specifically begged Trump to not put this guy up due to his skeletons and paper trail. They could have easily rammed through another Gorsuch.