The problem with extending the hours is that the vendors are extremely tired after standing on their feet from 10am - 6pm. Many of the booths are worked entirely by a couple people, and thus they are locked to that booth from the time the hall opens until after it's closed.
Also, many of the exhibitors are also gamers, and closing later, even for one day would mean cutting into their possible evening plans/games.
I realize a lot of people didn't get to spend as much time in the dealer hall as they would like. Personally, I didn't get to see all of the dealer hall this year, BUT I think extending the hours would only make it harder on the exhibitors.