I'm curious why the author's agent didn't just approach other mainstream publishers with it. After all, if he's a critical and commercial success with HarperCollins then it's very likely that someone, somewhere will take a serious interest. Now I know that the publishing world has conglomerated a lot in the last 20 years...but still there are other players.

edit: and editors at other imprints owned by the same group will often take a different view.