Because Hollywood hates movies that are made to make you think... and LotR was definately one that made non-Tolkien fans think about just how silly this whole Fantasy thing is...
Hell, after watching this, my Father has even decided to read Tolkien. It made him think for a moment that perhaps my life-long hobby is not so stupid after all
It's going to win visual effects for certain... LOTR said to Harry Potter, "I'll see your cave troll, and raise one balrog."
And that was JUST in the Mines of Moria. Add in everything else, from subtle body-sculpting of hobbits and dwarves, to magical fireworks, to brilliant modelwork in Isengard, and you've got Harry Potter beat hands down.