Canon and other lens makers routinely retire lens models and replace them. This one would have been retired anyway, no doubt. It was retired just about the time when people were switching from film to digital. A lot of lenses were retired then (and replaced with spiffier versions with IS). The slow-selling models were not replaced. This was probably one of them. It took them 13 years to sell 5,000 lenses. Not very profitable, lead or no lead.

Canon probably made a decision to replace the fabrication processes using lead with some other technology. When your fab processes change, your design usually follow. Lenses with small markets could well not be justified to go through redesign and probably retooling.