Probably because the vast majority of Apple TV users don't need or use the optical audio out on the 2nd and 3rd Gen Apple TV's. HDMI is now the standard and supports audio and video through a single connector.
I did use the Apple TV optical out for several years by connecting the HDMI to my TV and the audio cable to my older non HDMI receiver. I often had issues with the audio being out of sync with the TV image so I finally bought a new AV receiver with HDMI input outputs and no longer need to use the separate audio out on the Apple TV.
Its unfortunate for those that have older home theatre systems but it adds cost and complexity and Apple likely decided its no longer worth it on a new device launching in late 2015. Technology progresses and eventually we need to upgrade.