I guess Marc Platt opted for the movie musical.
Originally, WICKED was to be a straight movie. Then Stephen Schwartz came along and Platt agreed to make it a Broadway musical. Platt had always wanted still to do the novel as a straight movie and de-kiddiefy what the musical did to it. Perhaps he has since changed his mind?
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle