After some googling, looks like the answer is: it depends :) E.g. in this article (https://showbusinessinsights.com/2014/06/24/the-most-stunnin...) 5 of 6 boxes are in the middle, the only one at the side being from a British theatre. So maybe this is more of a British tradition? I guess both have their advantages and disadvantages - at the back the occupants have more privacy and a better general view, at the side it's easier to build a separate entrance, and the box is closer to the stage.