Deadline describes why Lurhmann chose Edgerton:
Luhrmann ... confirmed he'd found his man: "In casting Tom one had to find an actor who could credibly be (as Fitzgerald describes him) 'one of the most powerful ends that ever played football at New Haven,' had five-star acting chops and in the big dramatic showdown scenes between Gatsby and Tom, hold the screen against Leonardo DiCaprio, in the appropriate age group. Any wonder, it has been a long and thorough journey. The simple truth is that Joel came into our rehearsal space in New York and fulfilled all of the above criteria, and then some."
Read the full story at Deadline.
This article available online at:
http://www.theatlantic.com/entertainment/archive/2011/05/casting-the-great-gatsby-joel-edgerton-to-play-tom-buchanan/239090/