I wonder if they’re trying to keep Benedict’s role a secret? I know that the cast was announced long ago, but just a few days ago Jon Hamm was on a talk show and said that he wouldn’t reveal who was the voice of God because he wanted it to be a surprise. And we haven’t seen Frances McDormand at these events or doing interviews about this, so maybe they figure that people really don’t know who’s in it.
According to the reviews, BC is barely in it. It's just a cameo. The same with Mcdormand as God but she is also the narrator (and the voice over was criticized for being overly obvious with the typical problem of the author of the book doing the adaptation and wanting to include every description in the narration).
It sounds to me that BC's inclusion was very late in the production and the announcement and all that press is just Neil Gaiman and the producers wanting to exploit his name. Gaiman recently suggested that BC also did mo-caption (he said he did more than just the voice work) but I seriously doubt it. He is just really good promoting things.