I don't know in a referenced way, but at the time was spending a lot of time on dpreview, that was not the only lens discontinued when they weren't allowed to use lead to dope the glass, and most of the very highest quality glass did.

Of course, low demand explains why they did not make greater efforts to build a replacement with the new restrictions, but regardless of other motivations the EU lead regulation would have made them discontinue it, so it seems unnecessary to look harder for causes.

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.