There is an objective reason why societies grow more liberal, though. Namely, conservative ideologies function off of exclusions while liberal ideologies typically function off of inclusion. As conservatives move to the next group to alienate, the previous group becomes accepted by society and the diversity of that group increases. Any population whose ideology requires the exclusion of subgroups will always shrink as those subgroups are removed.
There is an objective reason why societies grow more liberal, though. Namely, conservative ideologies function off of exclusions while liberal ideologies typically function off of inclusion. As conservatives move to the next group to alienate, the previous group becomes accepted by society and the diversity of that group increases. Any population whose ideology requires the exclusion of subgroups will always shrink as those subgroups are removed.