That is a moral judgment of the two political movements themselves and then asserted as fact. There is nothing inherently morally superior about progressivism than conservatism. You've asserted objectivity on a fundamentally subjective stance.

Let us say for example that Conservatism favours the traditional family unit whilst progressives favour a free sexual market. I would fit broadly into the second camp, but that isn't an objectively more moral position to take. There are valid arguments to suggest children may suffer from high levels of divorce across society or be at a disadvantage from one parent families.

So even though I am a liberal/progressive by nature, I wouldn't claim to have objective moral superiority on decisions taken across society... that's a dangerously high road to take and a precarious position from which to fall.