You pretty much covered it. The reason iron is used is that it is less expensive, which to the best of my knowledge is the ONLY advantage to iron. But it's a false economy in my book. Iron can be a costly nuisance/headache to maintain in the long run. Also, for an external ballast/bolt on keel, the iron keel will require more volume (thickness) than a comparable weight lead keel, so there will likely be diminished performance at the same draft and/or the designer will have to resort to keel appendages like wings, etc.
I think, however, that you are mistaken about the move away from lead. Most quality builders continue to use lead for ballast and would not even consider using iron.