Why is it that Hodges has a spot in the opening credits theme song but David the coroner doesn't?

Hodges isn't always in every episode but David is in nearly every single one. Yet he only gets "special guest star" during the guest credits after the first commercial break. He is not a "guest star" when he is in 99% of the episodes. Even Mandy is in most of them and she doesn't even get the same credit recognition that David does yet her job is equally important and she gets the same amount of screentime.