think it has to do with Application
If you were making on a production line type of scenerio you'd want to stamp out as many of the "same" quilt as you could to save time/money. Your customers are many different sizes, shapes, sleeping postitions, etc you make something that will work for the majority. The rectangular shape will work best for the majority.
Now, for DIY. You are making it just for you and it's a "custom" piece anyway. Why not do the taper, maybe trapazoid style with greater taper towards the legs and wider at the shoulders, but still football shape.