Omission In Ariel's House

The exclusion and scapegoating of others is a common dynamic in modern society. In Ariel’s house, the dog is blamed for all mishaps. In English class, Dave is blamed for many things. In America, the President is often blamed for the troubles of the country. None of these people may be directly responsible for bad things, but it is easy to single them out as a target of ire.
The omission is significant and intentional. The purpose of the lottery is vague because it represents the old traditions that people hang on to, despite being meaningless. The lack of history emphasizes the pointlessness and brutality of the ritual. The townspeople do not appear to know the purpose of the ritual, but they participate dutifully because it is a tradition.