Why doesn't a parallel argument apply to material and scientific progress?

Presumably because it is possible to objectively assess the degree of material and scientific progress (whether they are good is another matter). We can tell that our current knowledge is better because we can say why it is better. If there were no no epistemological progress, LW would be in vain!

So presumably the argument that there is no moral progress hinges on morality being something that can't be objectively arrived at or verified. But examples of rational discussion of morality abound, not least on LW. If we can explain our morlity better than our predecessors we are justified in thinking it is better. (But progress in morality is not quite the same as progress in values. The values might be remain the same, with moral progress consisting of a better expression of those values).