This makes us open to relativism. Considerable social change in our own lifetimes, including social mobility, might have the same result. Mannheim's classic work in the 50s and 60s foresaw a lot of this. I suppose it is often discussed as a consequence of secularisation too. Instead, several equally plausible sets of ideas arise, each with their own definitions of truth and validity.
Postmodernist ideas express another dimension.
Then faith in these 'metanarratives' began to weaken, partly because social life seemed to get complex, as above, especially with the rise of the mass media, which constantly brought us new knowledge about social differences. As a result, it was time to celebrate other narratives, lots more narratives, lots more alternatives. If this is the source of the philosophical or intellectual roots of relativism, there are solid commercial factors too. Consumer-based industries especially like frequent changes and alterations, new fashions and trends, breaking down barriers and distinctions, and opening up markets. The tremendous drive towards celebrating difference, especially in terms of individual identity weakens shared values.