A couple of reasons. 1) There was too much electrical running thru the existing joists for me to tear out and redo. I think darn near all the electric in the house converge and go thru the joists of that room. That would have been a nightmare. 2) Running the joists perpendicular kept my spans shorter so I could use smaller joists. Tucking into the bays as per Rod's book would have given me a bit more headroom, that is true, but acoustically the inside out ceiling will be just as "high" as if I had run the inner joists between the existing.