Whether or not an election has been fair has criteria that can be objectively evaluated. With “moral goodness”, those criteria do not exist.

A moral relativist does not have explain why moral progress is good, because they disagree that you can even really define moral “progress”. At best, there is moral change, and society always catching up to whatever that change may have been. Mortality is an inherently subjective concept, and therefore what is considered morally good can change, or rather: the ‘moral goodness’ of something changes over time. Someone in the past may have fully believed their actions were morally just, despite us viewing them as monsters now. And perhaps things we now consider to be morally just will be considered morally reprehensible in 50 years from now.