LOL!
I'm not talking about the whole films, just the Hobbits and Orcs.
It's like anything. The more you see it the more you become familiar with it and the less impressive it becomes.
The first time you see a Centaur in the narnia films you think wow that's so cool. By the end of the film you've adjusted to seeing it over an hour or so and so that moment has gone. When you see them in the next film it becomes almost an afterthought. Oh there's a Centaur.
Human nature is to always look for the new and be interested or in awe of it.
One of the best things that happened to the Harry Potter films is as the series progressed they started to get darker. Had they all stayed like the first one people would have lost a lot of interest in them.
With the PC film it needed to go slightly darker to reflect the time and how Narnia had changed. When the kiRAB are walking through the forest have broken trees where a battle had been at some point. That would straight away turn it from being a lovely walk through a forest to something a bit less pleasant. Even if you don't know what caused or why there was that damage you know it was likely to be something bad.
It would of also added for the return of King Peter. He has returned to Narnia and now it is time for the darkness to turn back to light. It would be mirroring the events of LWW. Aslan has returned and now good shall prevail.