Maybe because that's the way the author intended them to be read?
From Wikipedia:
To make the case for his suggested order, Gresham quoted Lewis' reply to a letter from an American fan in 1957 who was having an argument with his mother about the order:
“I think I agree with your order [i.e. chronological] for reading the books more than with your mother’s. The series was not planned beforehand as she thinks. When I wrote The Lion I did not know I was going to write any more. Then I wrote P. Caspian as a sequel and still didn't think there would be any more, and when I had done The Voyage I felt quite sure it would be the last, but I found I was wrong. So perhaps it does not matter very much in which order anyone read them. I’m not even sure that all the others were written in the same order in which they were published.” (Dorsett & Mead 1996)
In the HarperCollins adult editions of the books (2005), the publisher asserts Lewis' preference for the numbering they adopted in a notice on the copyright page:
Although The Magician's Nephew was written several years after C.S. Lewis first began The Chronicles of Narnia, he wanted it to be read as the first book in the series. HarperCollins is happy to present these books in the order which Professor Lewis preferred.
Maybe he didn’t want to diagree with the little boy in a fan letter? He seems more attached to the idea that there is no one right order.
I like the chronological order for second readings, but for first- the published order just feels more organic. There’s more wonder and mystery to it. Reading The Magician’s Nephew first piles a lot on to you. Aslan as Creator, the whole Adam and Eve thing, the wardrobe.
You move onto TLTWATW and you know exactly what’s going to happen as soon as the wardrobe pops up again. You don’t have to wonder why this Aslan character has such an effect on everbody. Boy seduced by witch, big whoop- didn’t I just read this plot? It works better placed further away.
I always kind of liked the bookending of Narnian history in the last two books. It heightens the sense of falleness in The Last Battle. I don’t think TLB has quite that contrast placed after The Silver Chair, which also concerned itself with a fallen society.