I don't know why they changed it, but I can tell you why this wasn't the case in the OT: because it's self-defeating and pointless. Pointless, pointless, pointless. It would be like giving Dumbledore a large staff instead of a wand. Or having the film adaptation of "Return of the King" decided by a swordfight between Aragorn and some physical form of Sauron. I believe they actually considered doing that. Luckily, they didn't, and the LotR films are still a good example of a respectable film adaptation for a book that's difficult to transfer to the big screen without angering lots of fans.