Being a user forum no one here can answer "why" with any sort of confidence. My theory would be that since the ATV is designed for tv/movies and optical/toslink cannot support the new formats found in tv/movies (like 7.1) it was dropped from the new devices. In fact industry wide you will probably find less and less of the older Optical only devices as they are replaced with the industry standard audio/video via HDMI. but that's just my 2 cents as a user...
but their are work arounds for users like us who want to continue to use our older equipment - your tv may have optical out so you can run HDMI to tv and then optical to your receiver. You can by HDMI/optical splitter. You can upgrade your amp to be compatible with the new device you want to add to your home setup...