My guess is that it's because people are much more likely to hold a book by the bottom than by the top. Either because you want to avoid wear (as mentioned abov

My guess is that it's because people are much more likely to hold a book by the bottom than by the top. Either because you want to avoid wear (as mentioned above) or because you don't want your reader's hand to be obscuring the text. (Both of which also explain why the outer margins are larger than the inner margins.)