Because Presence (the album Achilles is on) is the most overshadowed album in the history of music. When Zeppelin released it, interest in older rock bands was at an all-time low. Punk was hot, Zeppelin were not. And Presence still doesn't get the praise it deserves. It is their most consistent and progressive album.
And, they only released one single from the album. Achilles wasn't on it, probably to preserve the awesome experience of listening to the whole album.