Good question. As an artist, perhaps he feels he has too much to say to "just be funny", and comedy tends not to be taken seriously - they certainly don't win awards.

I can think of a few actors who started in comedy and then got into more serious roles. Some still dabbled in comedy, others never looked back.

Michael Keaton went from Night Shift, Mr. Mom and Beetlejuice _to _Pacific Heights and The Paper. And then drops in on The Other Guys.

Eddie Murphy is a terrific actor, but it was almost impossible for him to break out of comedy and be taken seriously in serious movies/roles. He may finally be being taken seriously as anything more than a comedic actor with Mr. Church, even though movies such as Life and his deft portrayal of so many different roles within the same movies such as Coming to America or The Nutty Professor demonstrated his range.

Denzel Washington, far as I know, avoided comedy altogether, ensuring he only took serious roles. Didn't want to get stuck in a rut.

Will Smith and Jamie Foxx have done excellent jobs jumping up from comedy to serious work. Jim Carrey as well.

These lists can go on. Comedy is fun, but for actors, there's more going on for them than just making people laugh.

Another issue is that funny people are serious and have dark sides. There's more depression and early/young/unexpected deaths among comedians (John Belushi, John Candy, Chris Farley, Sam Kinison...) than we might otherwise expect. Being funny is complex. Look at how George Carlin used comedy to make serious statements about society....

I think I'm rambling now. But, hopefully these are some ideas that might provide some perspective to your question.