Order of publication, for the reasons you both stated, and also because it is clear that Lewis' ideas were developing as he went along. For instance, in "Lion" it is stated that the Witch is a daughter of Lilith, while in "Nephew" she is shown to be the evil ruler of a parallel universe. Similarly, in "Lion" she is a fairly straightforward analog of Satan, while in later books the character diverges from that model and become more nuanced. The idea of a multiverse is only present in "Nephew;" in all the books before, there were only two worlds mentioned.
Other things too. The evolution of Narnia in Lewis' mind makes sense if you read the books in publication order. If you put them in chronological order, it seems less like evolution and more like inconsistency.