I have no idea why they ever had the IsoZone dampners and personally I would not have purchased a Trek with them. They seemed like the weak link in the frame. If anything ever went wrong with one of the frames I would have thought this would be the point.
I would also think if you had such an awesome piece of new technology it would have first been used in the high end models and then trickled down. As for why they dropped it... don't know.