As an illustrator, art directors hire you because they want to give a project a specific tone, feeling and they need to be able to count on your work being a certain way for their project, campaign, etc. It's kind of like choosing a font. Imagine if you bought a Metallica CD and it was full of acoustic Bolivian folk music .. you would be kind of confused??? Art directors usually don't want to play "style roulette". This doesn't mean you can't do other styles...it just means when you present them to clients have consistency between the bodies of work. A number of illustrators, writers, and musicians work under various names for this vary reason.