I think that, saddly, many on the left have bought into relativism as a cheap way of winning arguments. It's not, it's mutually assured destruction of all value.
Basically they assert some kind of historical relativism to undermine the supposed absolute values of their opponents, hence destroying the possibility of any absolute values. But then they try to assert their own values on this scorched earth.
But once you've used relativism as a cheap way to criticise patriarchy or the PropertyRights, how do manage to reconstruct the idea that slavery is absolutely wrong or that women absolutely should be well treated?