why wasn't cindy in a very brady christmas

It seems that Cindy wasn't included in "A Very Brady Christmas" because her character wasn't developed enough in the show. The writers may have felt that the show had enough characters and storylines already, so they didn't need to add another character into the mix. It's also possible that the producers felt that Cindy's character didn't fit the tone or theme of the show, and they wanted to keep the focus on the original characters.