This isn't so much a historical question as a philosophical/sociological one. The reasons for the path to the afterlife in a specific culture would likely be a just question for a historian however the emergent pattern of the appearance of psychopomps in global theology isn't something that can be easily summarized. For example the cultural developments behind the appearance of Anubis in Ancient Egyptian mythology would be very different from those that lead to the appearance of Charon in ancient Greek.

The cultural influences and history/mythology that led to the assignment of a particular identity to the psychopomp in a particular culture (particularly an ancient one) is still something that would be very hard to accurately and concretely answer and this is multiplied exponentially when attempting any kind of global analysis. To even attempt to answer this question in any meaningful way would require expertise in basically all emergent global theologies and their accompanying cultures. Even then any pattern analysis would necessarily rely on sociological and philosophical thinking rather than historical data.

I've studied theology and the philosophy of religion reasonably extensively so if you like I can give you my answer from that perspective but it's not entirely appropriate for this sub so didn't want to give it in a top level answer.