I am planning to write a story where greek gods exist. But I can't figure out a reason why they would allow other religions to take over and people to stop worshiping them. And I need this for my story to work because it is set in the XXI century and these days the majority of people doesn't believe in Greek gods.
There could be a couple reasons:
1) Arrogance: the gods may have believed in themselves so strongly that they humans to form other religions. They may have thought that even if the other religions were present, because they were the 'first', that people would still contiue to worship them.
2) Carelessness: they may have forgotten how quickly time passes for humans and so not checked in on them for a while. For them they may have checked on their worshipers 5 minutes ago but for humans this may have been 100 years for example.
3) Free will: humans have free will and so there may not have actually been anything that they could have done to prevent it.
4) Entertainment: The gods may have decided to let the humans create these new religions so that they could take on human forms and act as these new gods/ messangers of the new gods. For example Zeus may have taken on the human form of Jesus and thus by creating a new religion got people to believe in him but just in a different strain of religion.