The point is well taken that it came out in a strong season. Also, it doesn't have the name recognition that Lion does. There are lots of people who have read Lion who've never read any of the rest, and some of those don't even know there are other books. Naturally, the first one released, with all the name recognition, would do inordinately well.
The producers have to recognize this and factor it into their plans. That's been clear from the start, when the title logo was a big, stylized "Chronicles of Narnia" with "The Lion, the Witch, and the Wardrobe" as the subtitle! Simple thinking would have made the classic name with high recognition be the big blaring logo, but they put some forethought into it and made sure to make the name of the entire series the prominent visual.
I've already said too much about my hopes regarding the film's fidelity to the written stories, so I won't repeat it here. I think as a run of produced films we'll see films through at least Dawn Treader. How well those films bring the stories to the screen is my worry.