Jonathan Haidt's formulation makes it simple, really. Liberalism prioritizes the moral themes of care and fairness. Other moral themes--loyalty, authority, purity-- must always defer to the first two (in a liberal society). But this is precisely what moral conservatives can not do. Harming or treating others less fairly is morally right for conservatives if it is done so based on loyalty (patriotism, religion, group identity), authority (religion, tradition), and/or purity (religion, sexual mores).