I think that the families who stay there are the ones who have been there for a while. Therefore, the lottery is a tradition to them like celebrating Christmas is a tradition for some. They simply got used to it. Plus, the children experience this every year as well and believe that this is a right and not a wrong.
Imagine if you were used to that and you moved to somewhere that doesn't do it, wouldn't you wonder why they
didn't do it, like they would wonder why you
did do it?
If I could interview the author, I'd definitely ask her how this tradition started in the first place, and why.
One thing I liked about this story was the fact that the author used the lottery, which the readers recognize as something different than how she used it in the story. This kept the suspense and curiosity flowing to the very end. Normally I can guess what happens at the end of a book/short story, but I could not for this one; all I could do was wonder and keep reading.