I've been reading some stuff about the EE being on two discs. I think that even if a BD has room enough to store the whole movie as long as the EE, there are two things that are mixing here into the final decission of splitting them.
One is the possibility of offering a better quality final product, with a higher bitrate and stuff. In spite off being very well known and successful, I guess Peter Jackson still cares for his movies (I can't tell whether he does for interest or because he's a LOTR fan himself).
The other thing that I think influenced the decission of having the movies in two discs is that EE are mainly made for LOTR fans. The content tries to be closer to the books (well... you know) and I think the packaging and organisation of the set of discs intended to mirror that. For every book of LOTR, JRR Tolkien established two "books" as subdivisions. Therefore if you read Fellowship of the Rings, you'll have Book I and II, TTT books III and IV and ROTK books V and VI. So I guess the possibility of offering the movies in two discs each is a nice wink to that.