In "The Lottery" by Shirley Jackson, why is it ironic that the villagers make fun of the next village for wanting to end the lottery?

It is ironic because the lottery is a senseless, barbaric ritual. It makes perfect sense to stop having a lottery. It is really Old Man Warner's stubbornness that shows real irony. 

Jackson never articulates exactly what the purpose of the lottery is. We only get a subtle hint about its origins and meaning from Old Man Warner. This ambiguity about the lottery is the point. This story is an allegory and the moral lesson warns against repeating mistakes of the past. More specifically, the lesson is that continuing a ritual or some traditional way of thinking simply because it is tradition is not always a logical and ethical practice. 

Old Man Warner gives us a hint about the reason for the lottery. He states an old aphorism, "Lottery in June, corn be heavy soon." The suggestion is that a sacrifice each year will lead to a plentiful corn crop. When someone notes that other towns are thinking of quitting the lottery, Old Man Warner calls them a "pack of crazy fools." He equates quitting the lottery with having the notion of going back to living in caves. The irony here is that he (the oldest man) is the one who thinks like a caveman. He is the one who is stuck in the past and is so resistant to change and progress that he cannot see, or refuses to see how "crazy" the lottery actually is.