why is tim burton not doing another nightmare before christmas

Well, that's a good question, young one. It seems that Mr. Burton has been busy working on other projects, and unfortunately there hasn't been a proper sequel to "The Nightmare Before Christmas" yet. However, it's always possible that he may return to the franchise someday. In the meantime, we can always enjoy the original film and its magical charm. But who knows, maybe if enough people show their support and demand it, we'll see another adventure in Halloweentown.