Getting fired from Apple was the best thing that could have ever happened to me. The heaviness of being successful was replaced by the lightness of being a beginner again. It freed me to enter one of the most creative periods of my life.
The most likely explanation is the most practical. 'Macbeth' is a very popular play with audiences. If you want to sell out a theater, just mount a production of 'Macbeth'. It's a short play, it's an exciting play, it's easy to understand, and it attracts great acting.
It's not like we have a formula, but I think one of the reasons this show has survived is that it has a big heart at its center. Other cartoon shows have people crap on each other and make racist jokes. But I don't think people tune in for that. I just don't think a show lasts for 10 years without a heart.