I don't know why they included Mantle. My understanding is that there were big expectations for him coming in to his rookie year. If you compare a few of the cards from the 51 set with the 52 Wheaties set, you'll see the same photos were used, particularly Feller, Musial, and Campanella.
I also came across three other athletes that were included in the set: Ben Hogan (I), Jack Kramer (H), and Bob Cousy (no letter identified):
http://www.legendaryauctions.com/Lot...Uncatalogued-P