When the album "No Rest For The Wicked" was released adding extra "hidden" tracks to the end of albums was the latest "thing." They are usually either an outtake or a song that doesn't fit stylistically with the album, thus they're are usually attached to the end of the final "official" track after a long space.
In this case, though, there is only a short space after the last "official" song, and the song that is "hidden" and uncredited is easily the best song on the album (imho). Without it the album is only 38:19 long, which is fairly short for a cd. And Hero makes a great album ender, whereas Demon Alcohol is not a very good closing track.
Additionally, it is more radio-friendly than any of the tracks that were released as singles. It is very accessible, yet it's still very Ozzy. But putting it at the end of another track virtually ensured it could get no radio play. Sometimes rock stations will pick non-single tracks from an album if they think it's worth playing. But in 1988 most radio stations were playing CD's, and unless they were given a single from the label they would be unlikely to spend the time to cue up a "hidden" track.
All of this makes me wonder if the producer or any of the musicians has ever spoken about why they chose to put such a good song as an Easter egg.