Maybe that's because instead of just CGing everything, the folks that worked on Narnia actually tried to have a good mix of CG, models, and the real thing. Narnia looked believable. ROTS looked just like what it was, like someone built a world inside a computer and then put some actors against green screen inside it. In three movies, Lucas didn't manage to do much of anything that was revolutionary (for him). That's the reason ROTK won Best Picture, to pay Peter Jackson tribute essentially for the entire LOTR trilogy. There's no way he would've gotten it for each movie, especially since they came out a year apart.

I loved ROTS as much as the next Star Wars fan (which is not very much), but Narnia was far and away more believable.