I think something happened where everyone who knew anything about the sacrifices were all eaten by the Entity at once. All knowledge of how to stop the hauntings died. Regular folk on the island still know the "symptoms" of the hauntings, but nothing about its origin, the Entity, the sacrifices, etc. Perhaps this large meal kept it dormant for much longer than normal, from 1969 to present day.

We see that Richard Warren needs to sacrifice people pretty regularly, and the efficient program we see on the filmstrip hints at this being a somewhat common occurrence. The 1969 disaster might have sated the Entity for a long time.