Oh yeah, it's totally a hodgepodge. My own personal Watsonian explanation is that no one from Archenland or wherever has gotten in without the Witch's say-so since the Winter began, or if they have, they were summarily killed. And if no one fit the four thrones requirement in Cair Paravel, and Narnia was known to be ruled by the Witch, then there'd be precious little reason to try. Particularly if the Witch made any sort of threats or demonstrations of her power.