Why Was Ed Norton Picked Over Mark Ruffalo In 2008?

The Huffington Post gives us the answer. From an interview with Louis Leterrier we read:

When people are asking me, because Mark Ruffalo is in this one, who's the better of the Bruce Banners -- both are great; both are fantastic -- but I actually wanted to cast Mark Ruffalo as Hulk and Marvel was like "No, you should get Edward Norton because he's more famous." So you see what I am saying? They are the ones who wanted Edward -- and I was thrilled to meet him and work with him. I wanted Mark Ruffalo. And they were like, "No, no, he just does smart, intellectual movies." Which makes sense, then and there in his career. But that's how I know him. We've stayed in touch and it's why he said "absolutely" when I offered him the part in this one.