what's the most commonly overlooked reason for forgetting?

I don't know if it most "commonly overlooked," but retroactive inhibition might fit the bill. This is when learning a new task interferes with remembering a previously learned task. This is when a stimulus leads to two different responses. First there is negative transfer, the first-learned response makes the second harder to learn.

Once both tasks are learned, each will interfere with the memory of the other. When the first task interferes with the memory of the second, that is proactive inhibition. When the second interferes with the memory of the first-learned task, this is retroactive inhibition.

A good example is driving on a particular side of the road. In the USA, we drive on the right side. In England, they drive on the left. If you learned to drive in the USA, that would interfere with learning this aspect of driving in England (negative transfer). Later, at times it will interfere with the memory (proactive inhibition). When returning to the USA, you would have problems driving on the right again (retroactive inhibition).

I hope this helps. Thanks for asking.