It is funny how speaker designs changed just in a relatively short time. According to all the stuff I have ever read, one of the primary reasons why pretty well all the companies got away from the large driver wide cabinets and went to the tall narrow deep cabinetry with smaller drivers was because of sound diffraction. If you put that old Pioneer speaker on a scope it would probably measure that many of the frequencies at various points, especially the highs, would be lost or at minimum be considerably affected by sound moving along the width of the cabinet. When all the companies that used the NRC in formulating their speaker designs(including Axiom)to provide a speaker that was as transparent, neutral and accurate as possible and the significant change that resulted, the current designs best met that requirement.
Whether bookshelf or floor standing, regardless of manufacturer it is pretty common across the board to see cabinetry not much wider than the speaker itself, all done to minimize diffraction. I am sure there are other reasons as well and with his wealth of experience, Alan Lofft could probably provide an in-depth explanation as to the history here.
In the end, having said all the above, in the real world if it sounds good to you that is all that matters.
