I'm going out on a limb here, but I say Potter is way easier to put in a theme park than LOTR.
Harry Potter would have been awesome even without Potter. The setting is half the charm of the series. It's a functional society with its own rules, stories and traditions. Atmospheric places with mysteries and quirks around every corner. Harry & co. are inhabitants of that world, but they do by no means define it. When reading the books, you get a distinct feeling that the Potterverse would go on as usual, even without Potter in it. It's a place any teenager would love to be a part of, even if none of the characters were in it. And above all, it all takes place in a world so similar to ours that is practially is ours, just with magic items here and there.
Lord of the Rings is... well, it's a little mishmash, really. You have the hobbits of the Shire. The dwarves in their mountains, the elves in the forests. The orcs in the gloomy-doomy-lands. The men everywhere else. On their own, these societies are exciting and fun, but it's not LOTR until everything acts toghether. The story is the most important, where a couple of characters from every little sub-society get together and go on a grand adventure. The world as we see it revolves around those characters. Apart from our group of hero travellers, the world is extremely heterogeneous. Elves play with elves, dwarves play with dwarves, and any member of either race is unlikely to meet any of the others. They are confined to their own lands, where their rules apply and everybody else are intruders. The orcs only show up to create trouble for the others. And the story of the books involves our characters travelling from one society to another.
How would you encorporate these settings in a theme park? For Harry Potter, it's easy. Take your average British village, turn the architecture up to 11, and add a few magic thingies. Boom, your main street is complete. The merchandise practically sells itself. As for any attraction, build it in a dungeon, add magic, and you're done. Your guests are in Harry Potter land. It might not be as easy as that to pull off, but conceptually, it's not a very big problem.
As for LOTR... what do you do? Where do you start? It's well established that all the sub-societies exist far away from the others, with lots and lots of scenic wilderness between. If you create an Elvish village, you can't have dwarves next door. It's firmly established that they hate each other anyway. If you want to go the safe route and create a setting of Men, well, there goes almost everything else. Involve the orcs, and all you can show is fighting. Peaceful coexistence between races is only shown in places where it's a day's ride or two between the settlements. If you want to create a believeable Middle-Earth setting, you have to focus on one race (hobbits, elves, dwarves, men, orcs) and forget all the others, or make a huge land where all the races exist in clearly separated areas, with as little co-interaction as possible (never mind the different architectural styles). There is some overlapping, where both Dwarves and Goblins live in the mountains, and there could be elven forests near the Hobbit villages, but it's somewhat hard to pull off.
On their own, neither of the races truly "are" LOTR, but when they act together, they aren't either. The story is all about the voyage between all the places, and is very hard to capture in a single location.
