It's very obvious that lens manufacturers design zooms with the 70mm as the dividing point. Thus, 24-70mm and 70-200mm. (There are exceptions, of course, but I think it's safe to say that 70mm is the most frequently used dividing point.) Even non-full frame lenses use the 70mm equivalent as the dividing point.
Does anybody know why? I did some search on Google but found no history behind it.