Possible reason why it was canceled after 2 seasons
Volume 2 of the Addams Family DVD set has a special feature that details the history of the show. In it, it is mentioned that both the Addams Family and the Munsters had decent, although not spectacular, ratings in their second seasons. Despite that, they were both canceled with no explanation from their respective networks as to why.
The second and final season for both shows was the 1965-66 season. It was the next season, 1966-67, when all network shows that were still shot in black & white switched to color. Could these two shows have been canceled because of the expense involved in transitioning them to color? Sets, wardrobe and makeup would have had to be altered, which likely would have been very expensive. Maybe their ratings weren't high enough to justify the cost of the transition, so the networks just canceled them instead.