Colin, the reason is readily apparent.. it's because the solder doesn't contain lead... Lead is a very ductile metal that flows well at a low temperatures, and is easily mixed with tin that's the entire reason it's used in solders in the first place. Although some lead free solders have issues with spontaneous tin whisker growth I don't see this as being any huge problem as they'll develop lead free solders and pre-cleaning or design changes that will compensate for the lack of lead.
Personally I think it's all blown out of proportion though, lead isn't THAT much of an electronics device, the chips themselves contain nasty metals and the epoxy boards themselves are the bulk of the product, not to mention the copper traces themselves, plastics and various other compounds (dissolved copper is particularly bad for plant life) The problem isn't that lead is used, it's that electronic devices aren't properly recycled and are landfilled commonly, if proper recycling facilities existed and it were common for devices to be recycled there wouldn't be a problem in the first place.