This is a good question.
There are three reason why the people in the town continue the lottery.
First, the people of the town have been holding the lottery for a very long time. In other words, the tradition of the town is to hold the lottery. This might not sound like a good reason, but in many places around the world tradition is extremely strong. Furthermore, within the story, the children are very much involved as well. This says that every adult there has participated in the lottery since they were children. From this perspective, tradition is even stronger.
Second, there might be some agricultural/superstitious belief about the lottery. Old man Warner recount a little jingle: "Used to be a saying about 'Lottery in June, corn be heavy soon." From this perspective, we can say that some people conduct the lottery, because they believe that there is some benefit to the community.
Finally, even though the text does not say this explicitly, there is a sense in which the people are afraid of change.