Why not use silicon, which also drops in resistivity when exposed to light? Silicon does even respond to most visible light as far as I'm aware, though not as strongly as CdS/CdSe due to being an indirect bandgap semiconductor. But other direct bandgap semiconductors exist, too, including ones without any highly toxic elements, like gallium antimonide or tin sulfide, or any of the more complex compounds used for thin-film solar cells.      

What gives cadmium compounds such an advantage? Probably because their peak sensitivity wavelength is close to that of the human eye and they do, overall, do a reasonable job of approximating human sensations of brightness.(Silicon is too responsive to IR and deep red light for some uses.)Plus, they've been in use a very long time and many of the systems installed decades ago are still working and meet their original specs when installed. That long successful experience might trump something with less time in practice (in some cases, anyway.) It's cheap and easy to make with low-tech.